Okay, so I just got DOSbox, (wanting to play Heroes), and got a few things mixed up, I guess. I accidently mounted drive D as the cdrom one (I needed drive C to be it, as Heroes was in drive C). Then, I was able to mount drive C as a cdrom drive as well. Then I mounted drive C as local directory. But now, when I double-click on a DOS game icon, an error comes up (in a normal WinXP window) that says something like some file in System32 (or something like that) is unsuitable for it. If I click ignore, it says it isnt responding, and it doesn't close (neither the error window, nor the game window). If I click close, it simply closes.

You cannot mix things up in DOSBos so it will have an effect on XP itself (the things you described as having done in DOSBox will have absolutely zero effect on XP itself). As was explained earlier XP and DOS don't mix so the error message you recieved is simply XP telling you not to run DOS games in XP. Once you have DOSBox installed and the drives correctly mounted you need to run the game from there. It will work.