There aren't that many processes you can kill on a clean windows install, but you can always stop unnecessary services, for a slight performance boost.
Here's a batch file I run before most games (in WindowsXP, obviously):
@ECHO OFF
REM Stopping non-essential services
net stop wuauserv
net stop W32Time
net stop Schedule
net stop ShellHWDetection
net stop ProtectedStorage
net stop Spooler
net stop dmadmin
net stop dmserver
net stop wscsvc
REM Stopping non-essential security services
net stop CryptSvc
net stop PolicyAgent
net stop seclogon
REM net stop SamSs
REM Stopping non-essential network services
net stop Browser
net stop lanmanserver
net stop TrkWks
net stop WZCSVC
REM Stopping non-essential management services
net stop SENS
net stop EventSystem
Beware that although this script leaves internet access intact, it does stop file-sharing.
If you don't need net access during gameplay, you can run this second script:
@ECHO OFF
REM Stopping more services
net stop Dnscache
net stop LmHosts
net stop Nla
net stop RasAuto
net stop RasMan
net stop TapiSrv
net stop Dhcp
net stop WebClient
net stop lanmanworkstation
net stop PlugPlay
net stop FastUserSwitchingCompatibility