Did you do basic troubleshooting stuff like-
updated scans for viruses, spyware, adware
have you run diskcleanup and Defragmented, if you havent in a while most likely the slowdown is coz of this, fragmented drive can be the cause of boot delays also.(you could defrag with a third party tool that defrags even the MFT and paging file if needed)
See if there are unnecessary items at startup and 'uncheck' them.
Uninstall programs you dont need. Gud Luk

Links on 'slow system'
http://www.microsoft.com/AtWork/getstarted/speed.mspx/
http://www3.telus.net/dandemar/slowcom.htm

Yep, sounds like it could be spyware. Download the MS Anti Spyware software it works well and its free.

128 MB of memory isn't a lot these days. If you want to see a performance boost, upgrade to at least 512 MB. Like everyone said; disk cleanup and defrag will help speed up your computer's performance.

Check out this tip from Microsoft.

http://www.microsoft.com/AtWork/getstarted/speed.mspx/

Agreed; a Windows XP system will run a bit slow with only 128MB of RAM. 384MB should be adequate if you aren't running any resource-hungry applications.

- You may be having lots of stuff that loads on startup, check what you have loaded whenever you start up. If you find applications that are not really necessary, get rid of them.
- Check your free disk space on the system partition
- Check your windows for Trojans/Adwares/Spywares

IF I could not reinstall Windows I would:

- Go to the Control Panel and remove all free software from Add/Remove Programs.
- Disconnect all network drives.
- Stop the print spooler service (start-->Run-->Services.msc or start-->Run-->net stop spooler)
- Turn off all sounds in XP; in the Control Panel it's Sounds and Audio Devices then Sounds and choose the sound scheme "No Sounds" . These tend to slow down the machine because it has to be able to run the sound at the same time you're trying to do other stuff like logon and get your UI loaded.
- Turn off Themes and Fast User Switching services. (start-->Run-->Services.msc)
- Run a defrag after ensuring you have at least 15% free on your system drive.
- Run regedit, back up the registry and then remove as much as possible from HKLM\Software\Microsoft\Windows\Current Version\Run