As said above, do a virus scan. If you've got something important you really need, back it up before on a brand new drive and dump the drive somewhere for if and when you need it (and try not to use it and when you do pull your hard drive out and run a live os to plug it into - the easiest way to do this is to burn a ubuntu ISO to a new DVD).
Once you've run a virus scan, backup anything you don't want to lose (after you've backed up everything you want or need to keep in the step above). You could also wait a while before accessing your files as there's a greater chance antivirus software has spotted the threat in 6 months if it's something quite new (they're more likely to be able to have heuristic detection too by then).
If you've caught a lot of viruses over the years then you'll be better off getting a separate machine for handling money and one for installing random things/visiting untrusted websites.
edit: every time when download some unusual software from an unverified site, I must first test it through Virustotal.
If you run software from publishers you don't trust, I'd recommend learning how to use virtual machines to sandbox them. It's really simple to do now (as long as your processor is compatible) and you can get free versions of most major operating systems now (including Windows).