installing program(antivirus )in the USB Flash drive

There are some type of USB flash that you could install Anti-Virus and some application on them. Note that you should install compatible version with those USB.One of these usb drive is called U3:

http://u3.sandisk.com/

McAfee have a product that you could install in your USB if it is U3:

http://home.mcafee.com/Store/PackageDetail.aspx?pkgid=269

But better solution is that if your Windows 7 support Bitlocker to go, then encrypt your USB with password and always have back up of your data

Also make sure your Anti-Virus is update and real time protection is on, whenever you insert a external devices such as Flash memory.

Not generally, Many AV cd's have the ability to boot from their cd to run a basic scan, so if you have the AV cd you would need to copy the contents to a usb stick and boot from the usb.

Presumably you have a virus problem on a PC?

yes exactly i have a problem and it keep me bothering since last week.....my problem is last week i had received a file came from another computer but a member of our home group via Skype...(that having a PC problem)....when i accepted it....suddenly my Microsoft office got a problem...i cant open it..when i opening it...it leads me to an error...and all  of the Microsoft icons in my start menu .....transformed into Microsoft word icons...how come it happens...i hope that i may get advices and information regarding this problem...thank you for the accommodation.....(preferably i am using Windows 7)