While a memory upgrade is the best way to improve the performance of your system, sometimes sluggish performance is due to a virus or malicious spyware. If you've already tried a memory upgrade and your system remains slow, or if you believe your system may be infected, a good antivirus program can help purge your system and set it on a path to better performance.
If you're using Windows XP, you will need to install an antivirus program. When evaluating different antivirus programs, it's rarely necessary to get all the additional features offered by antivirus software suppliers — most users simply need antivirus protection. Look for antivirus programs that update themselves regularly and that constantly scan your system. At a minimum, plan on running a full antivirus system scan every three to six months.