if you have xp follow the thread at the top of this page .or go to it with this link .
http://www.daniweb.com/techtalkforums/thread55459.html

It should not matter what operating system you are using so long as you still have the OS disks. If you have an OS CD simply place it in your CD drive and boot up your computer. It will either come up with a message saying "Press Any key to boot from CD....." or it will simply start booting on it own.

If you are using a Floppy simply do the same thing only with disk 1/boot disk in the drive.

Once you have booted from whatever device you are using it will pretty much walk you through the entire Process while installing it.

Thats Pretty much all you need to know as its not that difficult a task however when creating the partition for installing the OS on i suggest you partition at least half the hard drive too it. If you dont need a second partition just partition the entire thing.

Also bear in mind depending on how big your hard drive is partitioning half of it might not be enough so make sure it is at least 6gb

right i guess,not to be pickey but if by chance they are still using win95 ,and a lot are ,it will not boot to disk !!

A few things you can check first.
Defrag your drive, until it can defrag no more.
Wipe your temporary files (Disc Clean Up and Disk Defragmenter are both in Start>Programs>Accessories>System Tools). Don't wipe office files though, if you do you'll have to insert the install discs next time you use the program.
Search for a folder called prefetch in your system directories. Delete all the files inside that folder (Leave the folder, and make sure it is the prefetch folder). Your next (and only the next) login will be a bit longer than usual, but any programs you don't use anymore will stop hogging resources at login.
Run full spy-ware, ad-ware and virus scans.