Download Windows 7 Home Basic

Your PC maker has provided a means of reinstalling the entire system [possibly using a repair partition] so try this first as it's the easiest way to do it.  You will lose all user files & all installed applications and it may also remove all disk partitions. 

Using your existing Product Key you could create, then reinstall using, a Windows 7 Installation DVD/USB and I suggest a USB [see 1.4 below - you need to manually edit the DVD/USB that's created so a USB seems easiest but a rewriteable DVD would do it but I've heard that rewriteable DVDs are not as reliable as USBs in the long-term]

Requirements

-1 A Windows 7 / Vista / XP-SP2* PC on which you can use an admin-level account

[* XP requires additional components- see para 1.1 below]

AND EITHER

 - 2  Blank rewriteable DVD [see 1.4 below - you need to manually edit the DVD that's created]

OR

 - 3  4GB USB

Hazards

 - 1 This recovery method will result in the loss of all user files & all installed applications and it may also remove all disk partitions. 

Links to essential external procedures that must be followed during this recovery method

 - 1  Installing and reinstalling Windows 7

Links to advisory documents worth reviewing before starting this recovery method

- 1  Windows Help

- 2  PC manual

Procedure

1  To make an installation DVD or USB [I suggest using a USB stick as you'll have to do a manual edit to the Installation Disk so a USB is easiest - see 1.4 below]:

1.1  Download the correct ISO file for 32 bit or 64 bit Windows 7 Home Premium [see paras 1.2 & 1.4 below] from Official Windows 7 SP1 ISO from Digital River and the Windows 7 USB-DVD download tool  The download tool can run on Windows 7, Vista & XP SP2 but XP users need additional components that are listed, & linked to, on the download page.

1.2  You can only run a successful installation of the version of Windows [Home Premium & so on] that is identified on your Certificate of Authenticity [CoA] but in your case you will need to create the Installation disk for Home Premium then do a manual edit of the disk to make it able to install Home Basic [see para 1.4 below for that edit task].  The PC manual also needs to be checked as, if the PC is only specified as being capable of running 32 bit Windows 7 [“x86” but often referred to as x32], then only a 32 bit [x86] installation will succeed.  If the PC is specified as being capable of running 64 bit Windows 7 [“x64”] then a x64 installation would normally be chosen.  If you have any access to the system at all at this stage, then you can check the current version at Control panel, [set it to View by - large or small icons rather than Categories], System - see the entry at the top for the version & the entry lower down at System type for 64 bit or 32 bit.  Your Windows 7 CoA Product Key is valid for 32 bit & 64 bit versions so you can choose which one to install.  Your Windows 7 CoA Product Key is valid for SP1 even if you have not yet installed that update.

1.3  On another PC, using an admin-level account, install the USB-DVD Download Tool then run it with the ISO file stored anywhere on the system [except the target DVD / USB drive, I discovered, although others say that it can be on the target drive].  The result is a Windows 7 SP1 installation DVD / USB.  The USB-DVD Download Tool can then be ignored or it can be uninstalled.

1.4  Because you have a Product Key for the Home Basic version, you need to do a manual modification of the Installation Disk to make it into a multi-version installer [for Windows 7 Starter, Home Basic, Home Premium, Professional, Ultimate].  This is not an illicit procedure; you will use your existing Product Key and you will choose Windows 7 Home Basic during the installation procedure [your Product Key will only work for that version].  I have done that procedure on my Installation USB and it seems reliable.  On the Installation USB [or rewriteable DVD], go to the "sources" folder and delete the file ei.cfg - that is all you need to do and you now have a Multi-version Windows 7 Installation USB.

2  To install Windows 7:

2.1  Insert the Windows installation disk, (re)start the system.  As soon as the PC maker’s logo appears, set the system to boot from the DVD / USB drive as appropriate by pressing repeatedly on the Boot options key or the BIOS key identified onscreen at this point [or identified in the PC manual]. 

2.2  After several minutes [that can include periods with a blank screen & no apparent activity], wait during the “Starting Windows” screen then, at the first Install Windows dialog, select the language the time-currency format & the keyboard.  Then click Next. 

2.3  At the next Install Windows dialog, select Install now. 

2.4  Onscreen instructions will then guide you through the installation procedure.  There is more detailed guidance at Installing and reinstalling Windows 7.

2.5  At the end of the reinstallation, you may or may not have to activate Windows 7.  If you are reinstalling the version preinstalled by the PC maker then they might have provided for a self-activation process that you will not even notice.  If you have any doubts then you can go to Genuine Microsoft Software Validation afterwards to validate the activation / self-activation if you wish.

Have a read of this - http://answers.microsoft.com/en-us/windows/forum/windows_7-windows_install/when-can-i-download-the-windows-7-home-basic-iso/39eac399-3a12-46ed-91f4-93a300126e43

You may have a recovery partition to reload Windows from, or if not download and modify a W7 build as detailed in the answers. Hope it helps.