Is this thing extremely slow due to a cause, or just generally slow and needs all the help it can get? What kind of performance are you looking to gain?
You can disable some services for a little boost, you get rid of all the transparency effects, and eye candy for a decent boost. Defragmenting can help loading times. Increasing Virtual mem settings might help depending on your settings and how much ram you have
For info on services http://www.theeldergeek.com/services_guide.htm
For windows effects right click on My Computer > propertys > advanced, then settings under performance. Just unclick everything you don't want or need. You can change virtual mem settings one tab over under the advanced tab, but make sure you defrag before you increase the size.