The first thing to do would be to add more RAM. 512 MB is a minimum for XP but certainly not optimal. I recommend 1 GB.
Then, we might be able to trim down your Startup list and running processes.
Please click here to download and install the HijackThis installer.
Run it and select Do a system scan and save a logfile.
The log will be saved in Notepad. Copy and paste the log in your next post.
Do not fix anything