See if you get this message in Safe Mode at Startup? If you do not try isolating the cause via these.
310560 - How to Troubleshoot By Using the Msconfig Utility in Windows XP
http://support.microsoft.com/default.aspx?scid=kb;en-us;310560&FR=1&PA=1&SD=HSCH
How to troubleshoot a problem by performing a clean boot in Windows Vista
http://support.microsoft.com/default.aspx/kb/929135
Even though you are an Administrator on your machine, by design all users run with Standard User privileges. To get extra privileges, if you are not offered them via a User Access Control prompt, you may have to right-click on the .exe and look for the "Run As Administrator" option. This is referred to as running with elevated privileges.
I see lots of recommendations for programs like the following if you suspect malware as a cause.
SuperAntispyware
Malwarebytes' Anti-Malware