Thanks to Balaam's suggestion I've installed TortoiseSVN and moved Ice Slider into version control. Subversion does seem to be a fair bit less painful than CVS, given it didn't take that long to both set up the repository or to fix up the inevitable mistakes (such as checking in files that have no business to be under revision control).

I've also put across the (nearly) empty Project Crystal directory to see if it's painless to run more than one project in the same repository, and it seems to work. The starting revision number is already up to about 30 so I'm assuming it's based on the entire repository rather than the project.

I suspect it'll take me a while to figure out how to properly use the differences between the "trunk", "branches" and "tags" in the directory structure that the documentation suggested - I'm not sure what's standard practice for when to use a branch or what to name the tags - and I'm still unsure of the best way to backup the repository, but at least now I don't have to be so paranoid about breaking something with a sweeping architecture change.