The native firewall shouldn't slow down yr computer noticeably. If anything, it should only restrict what you can and can't see. But speed shouldn't be an issue.
If your PC is running a third party firewall product, you may want to make sure the Windows firewall is disabled so the two don't conflict.
Settings -> Control Panel -> Windows Firewall
It may be that your PC is bogged down by Adware/Spyware. I've found the following (free for personal use) products to be very good at keeping my PC clean:
Windows xp firewall does not slow a computer down, it must be something else. An alternate would be to get a router which sends out a fake IP address which is better than a firewall.
This app is light wieght while running, buit deep cleaning. This is what you will want to run since your system is lagging resourses. I also suggest hijackthis.
It will create a log file that you can post that will assist us in getting to the bottom of your issues. I can assure you it is not your windows firewall robbing resources. If you are having issues getting these or other applications running on your system, ook for windows washer to assist cleaning up any memory leaks to free up your resources.
Another thing to keep in mind, xp does not run well with less than 500MB, 256 will make it craw. More than 500 may be beneficial, depending on your system.
Try clearing your cache, that will slow down your computer as it collects unused data that you have downloaded over time. If you use internet explorer, go online and on the above task bar click TOOL, then INTERNET OPTIONS, then DELETE FILES in the Center, then CLICK IN THE BOX THAT SAYS "Delete all offline content", then click OK.
Depending on if you have ever done this before this could take from 5 minutes to 2 hours to complete. You should do this once a week minimum. If you have never done it or it has been a month watch how much faster the computer is. I restart the computer after I do it as that resets everything.