Doesn't have to be the Sasser worm. Could be bad RAM, bad spot on the hard drive or a new driver or program that isn't right. Have you tried booting into Safe Mode? Reboot and when you see the maker's logo or it beeps of it says press F8 to enter Safe Mode start tapping the F8 key slowly. Keep tapping until you see the menu. Choose Safe Mode (the top option). Choose your usual login and see if it's able to boot all the way. If you make it that far then:
Start, (All) Programs, Accessories, Command Prompt (Vista => right click and select Run As Administrator). Type (with an Enter after the line:
Under Startup tab, uncheck everything. Under Services, click on Hide Microsoft services then uncheck everything. OK and reboot. See if it will get into regular mode now. If it does one of the programs you unchecked is the problem and you just have to keep going back in to msconfig and turning 1/2 of them on at a time until you find the culprit then uninstall it.
There is a nice program from Avira that you can use to scan your hard drive for viruses even if you can't boot into windows. I believe it also let's you recover your data. You will have to have a friend burn a bootable CD for you then (after changing the boot order in the BIOS setup so that it looks at the CD/DVD drive first) boot from the CD.Download this file and save it:http://dlpro.antivir...m-common-en.iso
Then Burn Image or Copy from CD depending on the burn program they have.