I have one rather superannuated PC running Windows XP, which I refuse to upgrade to Win 7 for various reasons (not least being that I suspect it would run at the speed of glacial drift, if at all). Lately, it's been complaining at boot that the system registry is too large, and that future requests for registry space will be denied (and the offending programs no doubt will be executed).
So I did a little searching and found a pair of free programs by a programmer in Germany named Lars Hederer: ERUNT and NTREGOPT. The former backs up and restores the registry (not a bad idea when you plan to perform surgery on it), while the latter compresses it. In my case, NTREGOPT reduced the registry size by 75% (!!), from around a quarter GB to something in the 70 MB range.
Hmm ... wonder if it could reduce my size (weight please; advancing age is already attacking my height) (and preferably not 75%).