Dracon M'Alkir
Vault Senior Citizen

Note: This was a fair test. Both benchmarks were done in the same environment, aside from operating system. No hardware changes were made, and the latest drivers for both operating systems were used. These were the optimal conditions possible for gaming and benchmarking. (I did this on my own system using CrystalMark. Both copies were of the same version.)
Windows XP Service Pack 2
Windows Vista Service Pack 1
Comparisons:
CPU
XP: 57293
VA: 54583
Difference: -3.85% Slower
Memory
XP: 15243
VA: 15257
Difference: No Change
Hard Drive
XP: 16962
VA: 17772
Difference: +3.85% Faster (Thanks to ReadyBoost)
2D Rendering
XP: 22078
VA: 17897
Difference: -18.7% Slower
3D Rendering
XP: 35267
VA: 29555
Difference: -15.97% Slower
Overall Change: Vista is 7.41% slower than Windows XP on the same hardware.
Windows XP Service Pack 2

Windows Vista Service Pack 1

Comparisons:
CPU
XP: 57293
VA: 54583
Difference: -3.85% Slower
Memory
XP: 15243
VA: 15257
Difference: No Change
Hard Drive
XP: 16962
VA: 17772
Difference: +3.85% Faster (Thanks to ReadyBoost)
2D Rendering
XP: 22078
VA: 17897
Difference: -18.7% Slower
3D Rendering
XP: 35267
VA: 29555
Difference: -15.97% Slower
Overall Change: Vista is 7.41% slower than Windows XP on the same hardware.