To remove the list of recent sites you have visited, you could clear the browser history...however, that would completely remove the list of any sites you had visited.

First off, 256MB of RAM is not exceptionally high for windows XP. I'm sure you would see better performance if you had 512MB or more. (512MB is a comfortable amount)

You should check Norton antivirus autoprotect and make sure it is in Smart Scan mode, not Comprehensive file scanning mode. That should improve the speed of you computer a little bit.