Cleaning registry remains, defragmenting hard disks, and such will have a minor impact too. Things like file indexing services are practically useless, and I guess you could tweak off a number of other Windows enviroment services to fine-tune it, but that's neglible. Those would probably mainly affect boot times, anyways.
In addition to the RAM fix suggested, a dual core processor does wonders to smooth things up, in case you're annoyed of the virus scanner firing off in the middle of the porn movie causing a choke. For me it's been making a difference.
If you insist on disabling some background services for shits and giggles, msconfig can plug off useless trash running (
for a search on different programs to know which ones to turn off), and for managing background services through run > services.msc (
for a list of items).