I've found that those programs caused more problems than they fixed. I remember a program that unloaded unused DLLs from memory but somehow caused a permanent increase in idle CPU usage until the next reboot. Some free ones like CCleaner are popular though, but I've never used it myself.
I usually just tell people to get an antivirus program and occasionally use a malware scanner. You can get good ones of both of these for free. (try avira/avg/avast for antivirus, malwarebytes/superantispyware for malware scanner) Full system suites like Norton 360 usually cause more headaches and slowdowns than they are worth, as randomizer said.