Even with an older computer 3 hours seems like a long time. Generally speaking, the more files and folders that have to be scanned, the longer the scan will take. You could try doing a few housekeeping chores to get rid of clutter that accumulates on every computer.
From the desktop with no programs running, click start > control panel > double click internet options. A dialog box with several tabs will appear. On the General tab, click Delete Cookies, Delete Files (including offline content) and Clear History, one at a time. Answer OK/yes when prompted, restart computer. The scan could take quite a while (perhaps more than an hour.)
You can also run the Windows XP Disk Cleanup and Disk Defragmentation utility. Again, from the desktop click start > all programs > accessories > system tools. Select Disk Cleanup first. After a scan is done , you'll see a box where you can check mark a lot of different items. Check them all and run it. The Disk Cleanup scan can take an hour or more. When finished, go back to Disk Defragmentation and run it. If you've never done a defrag it could take a few hours the first time.
This Microsoft article gives more information.
http://www.microsoft.com/uk/atwork/get_more/speedupyourcomputer.mspx
And this article has a few more suggestions:
http://www.ehow.com/how_2054052_speed-up-slow-computer.html?ref=fuel
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