Disble these services:
Alerter
ClipBook
Computer Browser
Error Reporting Service
Human Interface Device Access
Indexing Service
Messenger
Net Logon
NetMeeting Remote Desktop Sharing
Network DDE
Network DDE DSDM
Network Provisioning Service
QoS RSVP
Remote Desktop Help Session Manager
Remote Registry
Routing and Remote Access
Secondary Logon
Server
Smart Card
TCP/IP NetBIOS Helper
Telnet
Terminal Services
Uninterruptible Power Supply(Don't disable if you have UPS with shutdown software)
Universal Plug and Play Device Host
WebClient
Windows Firewall / Internet Connection Sharing(Don't disable if you don't use a 3rd party firewall)
Windows Image Accusition(WIA) (Don't disable if you have a digital camera or scanner)
These are the most safe settings recommended. I am not responsible for any harm caused to your PC. Try disabling these and you will see boost in performance
Note: I have assumed that your PC is a simple PC and is not on a network or Office LAN type connections