First of all, if they've been fiddling with the BIOS, how about restoring it entirely? If that wasn't what you did. Second, you might wanna try out a harddrive speed benchmarking tool? Slow harddrive, slow machine... 400mhz and all sounds good so that's the first thing that comes to mind, I had a similar problem with my *erhem* replaced XBOX harddrive :whistle:
Edit:
You could try
this one for example, easy tool. Plenty of others there too if you want.