Ok, sorry if it changed anyone's file associations.
It didn't on my computer.
If it did on anyone's, right click the file and click choose program and you can set the default program for that file type.
Or as in Phil's screenshot, you can click 'change' or 'restore' or 'Advanced' as his has. I guess cuz I'm on xp home at the moment, it's restore instead of advanced.
I still use Imagine for my default, so it's as easy as going into Imagine's settings and select all files list to make Imagine the default viewer. Not all have my preferences though.
http://www.nyam.pe.kr/The newest version is in the first thread of this site I posted.