|I don't know if you can disable it but you can try installing different drivers:|
Open device manager (control panel--system--hardware--device manager)
Open 'mice and other pointing devices'
Double click on your mouse.
Click 'driver' tab.
Click 'update driver'
Check 'no, not at this time' in reference to connecting to windows update and then Next.
Check 'install from a list. . .' and then Next.
Check 'don't search, I will choose. . .' and then Next.
Uncheck 'show compatible hardware'
All of XP's native mouse support should now show. If it's a PS/2 mouse you can try the 'Microsoft--PS/2 compatible mouse' driver. There's another you could try. It'll be at the top of the manufacturer list--[standard mouse types]. Open that and chooose 'standard PS/2 port mouse'.
If you've got a USB mouse you may need to experiment with various MS USB mice, although they may all have support for those extra buttons.
When you decide what to install, click 'next'. You may get a message about that not being the best driver. Ignore that and install that mouse. You may need to shutdown windows and reboot before the new drivers take affect.
Oh, and I guess you could always use a mouse that doesn't have the extra buttons.