If you can boot into safe mode, do so with "networking" option during a reboot. Once done, you've have access to CD drive after safe mode with networking is compete. If you haven't yet, use the AV pgm. and do a scan, allow it to finish. using the CD drive, access any pgms. required. You should also be able to access the WWW, depending on your type of access. Try to access the WWW and able, then update the AV and then re-run a scan again. providing it finds anything take action. if nothing is found then try this, do an OS repair. You need the OS install disc, and select "repair" and allow it to boot into repair console. Once there, type: chkdsk /r and allow it to run, it will take much time, so enjoy whatever. Once, it is finished or requests your input, do so until done. Check the results. If nothing seems to work, then you need access to a "REPAIR DISC", which if not part of the OS install disc, then one generated for your OS(Vista, 32 04 64-bit), any similar Vista. This acts on its own to repair the OS of errors, issues, etc., and is different from OS recovery/restore or full OS(retail) discs. If you like, you can buy for small cost usually these discs from online vendors, just pick one to trust. Excluding any malware problems, these repair disc help recover your current install of OS issues and may a fix, thus a clean boot-up. You can generate a "repair disc" from another similar Vista PC under the control panel, see link.
Understand you want to "repair" NOT reload the OS, though it may appear that's what it's doing. Just use common sense. Also save the repair CD for next time.
You can create this System Repair Disc in Windows Vista or 7:
Click Start > All Programs > Maintenance > Create a System Repair Disc