|mmm...first off safeguard your data - all those important files, designs etc.... you wouldn't want to lose... Copy them to another (external?) hard-drive and/or optical media (DVD).|
Verify copies are truly accessible on at least one other working system.
If the system is OK as is apart from Visual Studio... I'd be inclined to:
a) Run a "full" virus" scan using the installed virus utilty; and also a freebie on-line on via Trend.com - Housecall. Ideally run both scans in safe-mode. And if you have a knoppix or ubuntu disk (linux variants) - boot with that and run a full on-line scan that way... "Anything" quarantined in any scan... delete entirely...
Also disable system-restore before running any scans using the utils installed and running via the current OS... Once system is "clean" - reboot; then re-enable system-restore.
knoppix.com and ubutu.com both have free .iso files to download; save to a hard-drive; burn to dvd; boot with that dvd.
b) Re-install Visual Studio.
Reformatting etc. would NOT be high on my list at this time; as if everything else is OK... then I can't see any benefit...?