As its name indicates, the Indexing Service indexes contents and properties of files on local and remote computers and provides rapid access to files through flexible querying language. This service is a major resource hog and should be disabled.

To disable the service, open up Notepad, and copy the following:
Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\cisvc]
"Start"=dword:00000004

Save and name the file something like XP_Disable_IndexingService.reg. Run the registry file you just created and reboot. You should notice an increase in system performance.