Ugh! talk about disinformation!
You didn’t mention how old the monitor was, and what brand it was, so I’ll do my best.
Using the monitor from your mac on a new PC should be no problem. There are exceptions, and I will list them in a moment.
For monitors, RAM, and Hard Drives, the type of computer doesn’t matter at ALL. For RAM and Hard Drives however, the motherboard need to be able to support them. But this is not a “PC vs Mac” thing, this is a “as technology progresses, all computer change over time” type thing.
But monitors have changed very little over the last 6 years, other than getting cheaper. If your monitor can do multiple resolutions (640x480, 768x1024, etc.) then you have a relatively recent monitor, and moving it to your PC will be no problem.
So if you have any 3rd party (non-apple) monitor made within the last 6 or so years, you should be in great shape. If fact, it’s prolly connected to you mac right now with a cheapy mac adaptor. Just take the adaptor off and you can use it with your PC.
Exceptions:
If you have a really old Apple monitor, and it doesn’t do multiple resolutions, then it probably has a proprietary connector on it. Sucks to be you.
You have a really new apple monitor that has the single cable connector (for power/video and USB?) then you have a proprietarty connector, and again, sucks to be you.
Now, video cards. If the video card in your Mac is a PCI one you might be able to use it in your PC. But it probably has 8megs of memory max on it, and if you say your computer is old it is probably closer to 4 megs of memory.
Your best bet is to go and get a new AGP video card for your PC. You can get a nice 16meg video card new for about $50 new, and probably less used.
One more thing: Don’t be afraid of buying used hardware, especially video cards or RAM or anything without moving parts. If it worked previously, it will most likely always work.