So, you are in a bind. Hard for you to go back to them and say - your PC doesn't work. Even though that is true, it makes you appear a tiny bit less capable than they might hope.
Two suggestions - as I already mentioned, booting a CD of a portable Linux will get you into the PC so you can extract what you need to a USB stick and hence onto your own PC to work on as needed. This won't help if it turns out there is software required to work on it, which you don't have, or it will not install or will not run on Win7/8 whatever you have personally. At least, if you do have to go back and say 'this PC has been all shook up and never started for me, give me another'....then you have shown competence in getting well past that first obstacle.
So far, you have not told us much about the PC apart from it runs XP pro - is it a box or a portable? Have you tried another keyboard and mouse as suggested? Furthermore, since this PC is probably quite old and has had no power for quite a while, I would not be at all surprised if the CMOS battery was failing to keep the PC bios correct, and that will very, very often lead to a non-booting situation. I could even demonstrate that with an older PC of mine. Unfortunately in that situation you do need a certain familiarity with what settings the bios might require, but at least, the date and time, the right HDD at the top of the boot order, the right drive channel for that drive, settings for keyboard/mouse/usb are all possible things to check.