XP & Windows 7 behave different?? (no replies)

I am just finishing up a EckerTech RepRap with a RAMPs 1.4, and Repetier V0.95F.

On my desktop with Windows 7 everything seems to work fine but when I switch to my laptop running Windoes XP, everything works EXCEPT I can't "home" the Z axis - the other axis home okay.

I am brand new in RepRap but been around computers for a long time and I don't understand why just this one bug when switching OS.

Any ideas?

Thanks!