Built my sister a new comp, but kept old HD

[fsstrike]

Yesterday I built my sister a new computer:

A64 3000+ 754
DFI Lanparty NF3-250GB
512mb Kingston Ram
Antec 380W PSU

All other parts, I took from her old computer:

GeForce3
Wireless Network Card
CD/DVD ROM Burner

I havnt had very much time to play with her new setup, but from what I can tell it doesnt seem as fast as it should be. However, I think the reason why is because I never completely reformatted her hard drive, everything is still on it from the old computer. I simple "repaired" it. The computer does not seem slow by any means, but I am simply wondering if not reformatting it is a bad choice?

 

[IEC]

It would be better if you reformatted, but you can simply clean the registry, defrag, and delete unnecessary files to run faster.