To disable unneeded startup services for a safer, faster XP, use the "Services" Admin Tool (Control Panel > Administrative Tools > Services). If you are a single user of a non-networked machine, you can disable the following items, with no ill effect.

Alerter
Clipbook
Computer Browser
Fast User Switching
Human Interface Access Devices
Indexing Service (Slows the hard drive down)
Messenger
Net Logon (unnecessary unless networked on a Domain)
Netmeeting Remote Desktop Sharing (disabled for extra security)
Remote Desktop Help Session Manager (disabled for extra security)
Remote Procedure Call Locator
Remote Registry (disabled for extra security)
Routing & Remote Access (disabled for extra security)
Server
SSDP Discovery Service (this is for the utterly pointless "Universal P'n'P", & leaves TCP Port 5000 wide open)
TCP/IP NetBIOS Helper
Telnet (disabled for extra security)
Universal Plug and Play Device Host
Upload Manager
Windows Time
Wireless Zero Configuration (for wireless networks)
Workstation