A computer should be rebooted at least once a week. Some software we run on our computers do not completely release memory efficiently and slowly eat up the memory on your computer. The memory can be regained by rebooting.
Another reason computers run slowly is virus and spyware activity. If a reboot does not help, download Spy- Bot anti-spam software. It can be easily found by doing a search on your favorite search engine. Installing this an running it will clean a great number of spy ware programs from your computer. Make sure you do a search for upgrades before doing your first scan. In conjunction with Spy-Bot, you may choose to install and run AdAware. This is also another good program for removing spyware. It will remove some spyware missed by Spy-Bot.
Finally, a good virus protection software is essential to keeping spyware and viruses from getting on your computer in the first place. I recommend Norton Anti-Virus. If you have kids, you may want to upgrade to Norton Internet Security. It contains the anti-virus software plus parental controlls for blocking kids from visiting undesirabe internet sites. If you already have an anti-virus program installed, run it after running the spyware removal programs.
After taking these steps, you may find that your computer runs a little faster. If not, or if you cannot install or run any of the programs above, a virus may have disabled these security features and you may need to consult someone about restoring your system to factory condition.