If you've already got XP install it and go to
www.blackviper.com and follow the guide to reduce processes.
Turn off ALL auto updaters and scheduled tasks and schedule a day or two a week to do it manually. It takes about 5-10 minutes.
If you're behind a router you have no need for antivirus, antispyware, etc. running all the time unless you have very loose security settings on your browser, download a lot of potentially dangerous files from the internet or click attachments on e-mails from unknown senders. Otherwise update on your update days and run scans manually once a week while doing other chores.
XP will run fine as long as you trim the processes, regularily delete temp and intenet files, defrag and otherwise keep a clean system.
