Dawn - Ccleaner is very popular and generally very good. As Midase says, I would update the the program first - it should check for updates automatically when you open the program.
There are several options down the left hand side. I would run both the 'cleaner' and 'registry' cleaners, first analyzing and then clicking 'run cleaner'.
Under 'tools' you can look through all the programs on your computer and then uninstall the ones you no longer use. Don't remove any programs you're not sure about. There's also a list of startup programs, again: don't remove any you don't know about but some things you may understand and not want.
Also under tools is a list of system restore points, I would only keep the top 5 (the latest one's) and then select all the others (click, on the 6th one, scroll down to the bottom, hold shift and click on the bottom one) and click delete. The old restore points aren't necessary to keep.
Midase talks about doing a defrag - you can either use the built in one on accessories on the start menu, or I'd reccomend Ausologics Disk Defrag
http://www.auslogics.com/en/software/disk-defrag/. Ausologics Disk Defrag is free and has an option to defrag and optimize - giving a better speed boost. It's also more up to date that the software built into Windows.
I hope this helps a bit. Be sure to let us know how you get on, and if you have any more problems or the computer is still a bit slow, let us know and we'll try some other things.
Jack