Plenty of free options. First I would ensure your anti virus is up to date (If you don't have one, get one, AVG Free is pretty common and reliable) Allow the software to do a full system scan (this can take several minutes or hours)
Next run a good Spyware program. Microsoft Security Essentials is a piece of software that has become more reliable and is free. Let that do a full system scan
Microsoft Security Essentials also can scan for viruses, so if you have neither Anti Virus or Anti Spyware program, get it.
After that is done, Defrag the hard drive.
I would also consider clearing temporary files, files still in your recycle bin and a great program to help with that is CCleaner (www.ccleaner.com) it can do more including removing invalid registry keys. This cleans up any reference in your system registry to any programs you no longer have, but your system is still looking for them.
The slower your computer the longer this can take but is well worth it.
Finally, you can run a CheckDisk (known as CHKDSK) from the command line. Let it prompt you and agree to running the first time you start your system. I had one laptop 2 weeks ago that ran very slow, the fix that put the icing on the cake was bad sectors in the hard drive identified by CHKDSK and was able to repair, after that, the system was humming along.