Anyone know of a freeware utility I can use to patch a file? I have a lot of users with version x of my software, and I need to patch the .exe to version y, whilst obviously being able to do some kind of CRC check on the existing exe so I know its legit.
Anyone know of a utility before I start coding one?

Patch Maker (http://www.clickteam.com/English/patch_maker.htm)

It can even read the installation path from the registry, as well as update more than one version of your software with the same patch.

PatchMaker is pretty good, though I found that the size of the patches created by it wasn't as small as it could have been. I ended up using it for my patches because its features were still pretty good for the money (i.e., free). However, you may also want to take a look at VPatch (http://www.tibed.net/vpatch). VPatch is an open source NSIS plugin, though it also works as a stand-alone app. So, if PatchMaker doesn't do everything you'd like it to do, you might be able to use VPatch, either as an out-of-the-box alternative or as a starting point for writing your own.

Thanks guys! Patchmaker did EXACTLY what I wanted to do, and did it very easily, but boy do they know how to put people off registering the thing? I went through about 6 pages of junk till I found the price, then It was £33 which seemed a bit steep. Luckily the free version was ok for me.
Highly recommended to all.