I agree that malware is the first concern. Using any of the popular search engines, look for "online virus scan" and pick an established vendor from the results. For malware, SuperAntiSpyware (http://www.superantispyware.com) is highly regarded and is reportedly used by Microsoft's tech support.
If you cannot get to any of the antivirus sites, that's a powerful indicator that your PC has been compromised. If this is the case, you might ask a friend to download a good antivirus/antimalware program onto a USB thumbdrive for you. (Avira's free application got a top rating in a recent issue of PC World, but everyone has their own favorite.)
If no virus is detected, the next step to isolate the problem would be to use a tool to see what applications are starting up automatically. Windows typically includes "msconfig.exe", a Microsoft program which allows you to choose a "Diagnostic Startup" (which runs with only essential services) or you can selectively disable various programs using the "Startup" tab. If you perform a "Diagnostic Startup" and your system does not have a problem, then you can experiment enabling different startup programs to see if you can isolate the one that is having a problem.
There are other good programs for checking your startup programs. I prefer AutoRuns.exe from what used to be Sysinternals before Microsoft bought them. The tool is now available from Microsoft separately or as part of the collection of utilities known as Sysinternals Suite. Go to http://technet.microsoft.com/en-us/sysinternals/default.aspx for more information or to download.
Hopefully one of these suggestions will either help you clean any malware from your system or isolate a corrupted installation.