Goto http://www.crucial.com/ you can scan your system from thier website. It will tell you how much RAM you have, the type, how many slots you have used and how many are free. Between 1 and 2 gig of memory should be sufficent for a Windows XP PC.
However there are other things that can contribute to a slow PC. I'd check your PC for Virus/Spyware/Trojan Horses first then check your HD for errors and fragmentation.
To Check your HD
1.Open Windows explore (Start > Run and type explorer)
2.Right Click on your HD (could be labled as Local Disk C: ) and click on properties
3. Click the "Tools" Tab
4. Click "Defragment Now"
5. A new window will open. Click your C Drive and then click "Defragment"
Note this can take sometime to run and you need atleast 10% Free Hard Disk space.
Once done re-boot
Has there been an improvement? If not do step 6.
6. Repeat 1 - 4 and click "Check Now" (above "Defragment Now") another window will appear tick both options and click "Start" Windows will reboot again this can take sometime.