I'm on a Windows XP home, would I be able to install the same game twice on the same computer? So I could run them separately?
The reason I ask is I have a game I love to PLAY now, and another one that I want to install and add a mod to it, is there a way to install without overwriting the other one?You might try installing to a different partition (if you have more than one DRIVE or partition available. You also might be able to change a folder in the directory path in order to direct the installation to a different folder.

I've never tried this. I'm pretty sure someone else here probably has. Thanx, I HOPE someone who has tried this speaks up, I am worried of overwriting the existing game, I have put a huge amount of time into it.....
I do have another partition that my wife uses, could try that, but again I will wait for some views on this....Not offended one bit. It should work, as long as you direct the software to install the game elsewhere than where it would install by default.

Let's say the software, by default, "wants" to install to:
c:\programs\games\killer_paperclips.exe

You can change the drive, or change the directory path (or both):
d:\programs\games\killer_paperclips.exe
or
c:\programs\test\killer_paperclips.exe

These are things I had in mind, but I will try things on my own computer that I won't necessarily recommend unless I'm sure.

What game are we talking about here?

Most games won't install a second time as long as their original registry entries are still intact.

What you could do is simply install the game once, as normal, to whatever directory you choose. Once the game is installed, copy, not move, the entire folder containing the game to a second location. Depending on the game, the game may still work, or it may not. If it doesn't, just delete the copied folder. If it does work, then add your mods to one of them. Then, locate the executable of the copied version and create a shortcut on your desktop or wherever you choose (so you don't have to go through the folders each time to start it).

If that doesn't work (because of registry issues), there's a longer route you can take, which is much the same, but will require TWO extra copied locations, and means you have to overwrite the game every time you wish to use (or not use) the mods.

Location 1: Original installation folder.
Location 2: Backup of original installation folder.
Location 3: Modded copy of game.

When you want to use the mods, copy the modded folder over the original installation folder. When you want to switch back, copy the original backup over the original installation folder. Depending on the size of the game's installed files, this could take a while. Alternatively, you could backup and copy only the modded files, if you know EXACTLY what they are and where they go. Using this method, you are still using the original installation folder, so the registry shouldn't give you any issues.Thanx for the tips quaxo, using that and Aegis advice I got it to work, nice to know it can be done if you know what your doing.....
Cheers! Thanks for the backup, Q! I hadn't properly considered the registry. I knew there was a reason I was hesitant to just say, Go full speed!

Glad you got it WORKING, fullbug! I've actually done this with quite a few games.

World of Warcraft - With the game, two expansions, and tons of patches... it's a lot to do on 3 computers (mine, my laptop, and my girlfriend's laptop since she plays, too). Luckily, WoW will copy over without having any registry entries and work just fine. It's a lot faster and easier than installing, downloading patches, and patching the game 3 times.

Freelancer - I had three folders with this game, each with different mods. No registry entries needed to run.

I'd go on, but I think you get the point.