These are 3 tools that I use. You can use them once, then harden the system, then uninstall them. Increased performance, privacy, security. Since you don't run them in the background, they wont use system resources. Remember that its a very good idea to make a system restore point before though.

xpy

Small tool which disables the default threats of a Windows XP installation. Besides disabling Windows and some of its components to communicate with Microsoft servers, xpy improves privacy settings and your system’s security.

xp-AntiSpy

The xp-AntiSpy is a little utility that lets you disable some built-in update and authentication 'features' in Windows.

Security & Privacy Complete 3

Security & Privacy Complete is mainly a security tool for Windows. It can disable all services which might be a security-risk, harden registry settings... Also included privacy features for Internet Explorer, Media Player, and of course: Mozilla Firefox.

They sound good, I'll wait until I have my new computer so I can test them without any chance of messing up my main machine...