Open the Device Manager and find the make and model of the disk. You should then be able to find technical information on the existing drive.
Electrically, you are unlikely to have difficulties (but it will probably need an ATA/IDE drive not a SATA drive). The only difficulty you are likely to find is the depth of the drive - this can vary slightly. When faced with a similar problem a few years ago, I took the old drive out and measured it.
If you are not sure what to do, take the laptop into a computer shop. They should be able to replace the drive and transfer all the files across, but this will cost more than simply buying a bare drive on the internet.