First Question: Do you have an OS that you can install on it (the CD's, I mean) 98SE, Win2k, Linux, XP, etc.
Second Question: Do you have drivers for the motherboard and video card and sound card on the computer for the OS above?
Third Question: You might be able to click cancel on the password screen and at least get to the files on the computer. This wouldn't be a long-term solution.
Otherwise, make a DOS disk to access the hard drive(s). I like to find out who made the drive (open the case and look inside) and go to the manufacturers website and get their software for wiping and partitioning the drive. However, I am pretty sure you can just type "format C:" and it will erase the drive (all data will be lost, though, including the drivers I asked about). Then you can install any OS you want on it.
Another option (but might not work with NTFS drives) (if you wanted to keep the data) is to download Knoppix and burn it to a CD (Tomshardware had a good article on it) and use it to delete the Windows directory and subdirectories.