For Disabling Hibernation
Open a command prompt and type
powercfg /h off
Press Enter

Warning : Do this at your own risk. If anything is not working for you, revert back to the previous state.

Click on Start Menu and Run. Type services.msc and click ok to open the service manager.

Unwanted Services in Windows XP

1. Alerter
2. Clipbook
3. Error Reporting Service
4. Help and Support. (if you do not need help)
5. Indexing Service
6. Net.Tcp Port Sharing Service
7. NetMeeting Remote Desktop Sharing ( If you are not using Remote Desktop )
8. Telnet ( If you are not using Remote Desktop )
9. Terminal Services ( If you are not using Remote Desktop )
10. Remote Desktop Help Session Manager ( If you are not using Remote Desktop )
11. Remote Registry ( If you are not using Remote Desktop )
12. Routing and Remote Access ( If you are not using Remote Desktop )
13. Performance Logs and Alerts
14. Print Spooler ( if you do not have a printer)
15. SSDP Discovery Service
16. System Restore Service (if you do not use system restore)
17. Themes (if you do not use Themes)
18. Wireless Zero Configuration (if you do not have a wireless connection)

Disclaimer : The list is not complete. I came up with this list from the experience i got from working with my 5 year old machine with 512 MB Ram and partially working motherboard.

1. Disable Visual Effects
   Right click on My Computer, select Properties, go to Advanced -> select Settings under Performance -> Select Adjust for best performance.
2. Set virtual memory to double of your RAM. Keep the Initial and Maximum size same.
3. Keep the size of your C: drive (drive where you install Windows) at 10 GB
4. Install all programs in D:\Program Files\
5. Disable sounds (control panel)
6. Disable system restore
7. Disable Error Reporting
8. Disable Remote Assistance and Remote Desktop if you do not use them
9. Remove Desktop background
10. Remove all unwanted startup programs
11. Disable all unwanted services.
12. Uninstall all unwanted programs.
13. Defragment all disks
14. Disable Desktop Cleanup Wizard
15. Uncheck “Allow Indexing Service to index this disk for fast file searching” after right clicking each drive.
16. Run CheckDisc once in a while
17. Unshare all useless shared folders and files.
18. Uncheck hide inactive icons.
19. Run diskcleanup once in a while
20. Delete system restore files
21. Remove Windows Messenger
22. Use classic start menu
23. Disable hibernation if you are not using that.  NB: Hibernation is a better method than sleep for saving time.