Flavius I disagree with you.
Resetting of the security settings will only work if there exists a restore point for the old settings-chances are this restore point no longer exists.
This actually is a secpol/Group Policy issue with the User account, and it is not necessary to download and run 3rd party apps (psexec.exe which is a Unix based command line tool for Windows NT and 2000 which mimics the User/Admin utilities already available in Vista, and is redundant i.e., list system info, kill running processes, dump even logs, change user passwords, etc.)
This is most likey not related to a Viral infection. For someone not Computer/software savvy troubleshooting and repairing secpol/Group Policy/User account corruption/errors is more time-consuming and troublesome than just backing up data and reinstalling the O/S when a restore point will not resolve the issue.
Again, I suggest the following troublshooting/workarounds:
1.
Take Ownership and check group policy settings in gepedit.msc (secpol.msc is not available in your version of Vista)
2.(Or use this workaround) disable UAC to install/uninstall the particular program, then re-enable UAC (as this is a very necessary security feature of Vista).
3. As a last resort, do a fresh reinstall of Vista (borrow a disk)