Hi mattie,
Start >run services.msc, scroll down to "Windows Installer" and double
click it. Stop the service if it is running, set the startup type line
to disabled. Click apply/ok and reboot.
Delete the contents of the temp folders, there may be a file in there
interfering with the setup routine of the installer. Get both:
C:\Windows\temp
C:\Documents and Settings\<username>\local settings\temp
Now reverse the first set of steps and reenable the Windows Installer
(You can just set the startup type to manual). Reboot.
Turn off all other running programs, especially your antivirus
software. Use ctrl+shift+escape, if necessary, to "end task" on
everything.
Check also:
Windows Installer Fix (Line 94)
http://www.kellys-korner-xp.com/xp_tweaks.htm
Troubleshooting Windows Installer
http://www.kellys-korner-xp.com/xp_w.htm#xp_wininstaller
Good luck