You should first do a spyware check, get super anti spyware free edition, its the best. Scan with that, do a virus scan with an up to date virus scanner and then uninstall any extras that will slow your pc, like google desktop, any internet explorer addon bars and anything that you dont know you have installed and dont use.
The more space you have free on your hard drive the better.
XP SP2 will run on 512mb ram as long as you don not have loads of extras installed. Uninstall any extra dell software, if you dont use it, that will kill a system.
Ben Citrine, January 2008