I installed Windows XP on Pentium 2 machines with no problems so you should try to install the OS on the machine but make sure the PC has at least 64MB of RAM and a 4GB HDD.
I installed Windows XP (and tweaked after installed it...) on a P2 (300MHz),128MB of RAM,4GB HDD,2MB video card and a simple CD-ROM drive with no problems.
You should definitely try to install Windows XP on the machine...if it hasn't even got another older operating system installed...why not try with Windows XP?
One IMPORTANT THING:When you want someone to help you , you give ALL the DETAILS, in your case THE PC'S SPECIFICATIONS.
For me... an Intel Pentium 4 is an old processor...so by using the word ' old ' different people understand different things.
Of course the really old machine is a P1,P2,P3...but what about a 486?..:))...
We can never give you the a really really good answer unless you give us some more details about your PC.
Processor type and frequency?(I suppose it's at least a Pentium)
Amount of installed memory?(I suppose it's SDRAM)
Hard Drive capacity?
Additional details are always welcome(if you can provide us with something like this):
-video card make and model