Where to download a bootable .iso image of Windows Vista (Home Basic):
http://en.community.dell.com/support-forums/software-os/w/microsoft_os/3317.aspx
... request for a new bootable DVD of Win Vista Home Basic:
For information on replacement software media:
http://support.microsoft.com/kb/326246
Tom Ferguson
PS
For greater clarity - from the top link to iso files:
Windows Vista Official Setup Files (Digital River)
Note these .iso files may be used with a Dell/non-Dell OEM product Key but phone activation is required (see note at end). This key is typically found on the COA attached to the base of the system.
Retail/upgrade product keys can be used with these .iso files and activated over the internet or phone. See note at the end for phone activation.