Have you used a virus scanner lately? When you install software that alters the registery, you have to reboot for the changes to take effect. You didn't mention the operating system, but everybodies first guess for something like this is WINDOWS.
Now, if you just installed an older version of Windows, might take it a while to download and install all the updates, security fixes, and basic crap that needs to patched, over, and over, and... Windows Vista? Dreading the future time when I have to "upgrade", don't know what kind of problems they're sell people this time, figure it's their worst effort yet. Don't think this 5 year old computer is up the task of running it.