REinstall the trialware.
Run regcleaner.
Uninstall it.
Reboot n deleate the program folder n related files installed in windows,system if any.
Think twice before deleating shared files.
Search for all keys names related to that sware.
Like if it winamp.
There wld be many keys left after the uninstall.
They can be named under winamp or company name 'Nullsoft' etc.
Deleate allall the keys named under the app name and yhe name of the vendor.
Now go for a fresh installment of the software.
Try to install it in some folder other than c:/Program files.
Most probably it will work.
Do backup the registry.
Regcleaner does it automatically.
Also before installing any software create a restore point
in system restore if it is enabled.
So that in case of emergency u can revert back to the restore point.
Other third party sw r also available for that purpose.
As for the virus problem.
Install NortonSystemworks n rescue on some friends PC.
Create a rescue disk with the latest virus definations..
Boot into real dos .
N run autoexec.bat from the rescue disk u created.
Scan the entire system for viruses.
Very time consuming but very effective cos it can detect infected
windows system files .
Most probably you explorer.exe is infected.
Try to repair if not then simply deleate.
Then reinstall windows.
N then install Norton or mcafee.
Remember to enable autoprotect in norton.