You have to tweak your VM's system settings to make the virtual processor look like the one on your old computer.
The important options are:
- IO APIC
- Number of cores
- PAE/NX
- The bit-version (32/64) of the OS type (this should not matter if your host OS is 64 bit)
I think Windows now automatically switches between HALs when it detects your CPU has changed, but this feature may not be there in Windows XP.