I couldn't figure out why the hard drive on our old home machine (and older Presario) was almost full. I'd starting getting memory space errors and went through the folders, stripped programs, and run cleaner programs, all to no avail. The culprit turned out to be the windows/installer folder – which had swollen to almost 20G. It seems that failed Windows XP SP2 installs can leave a bunch a of duplicate or orphaned .msp files. Many gigabytes worth!
The solution (PLEASE NOTE – I'm not a tech support guy; I don't necessarily advocate this solution for you and I don't know that it won't have a different effect on your computer than it did on mine… proceed with caution.) was downloading the Windows Installer Clean Up program, mostly to get the little program inside it Msizap.exe. Then, instead of running the Clean Up Program, you run msizap.exe from the command like so: "C:Program FilesWindows Installer Clean Upmsizap.exe" G! (Make sure to include the quotes and the spaces)
The G parameter tells msizap to remove orphaned files. Using the process with that, or with other parameters, can lead to all sorts of problems. (WARNING!)
I ran the command line as above and it freed up 17G. from the installer folder in about two seconds. Amazing. Everything on the machine is working right (so far!) and I'm not getting anymore memory errors.
But be careful; I found reference to this solution in a number of places (here are links to one of them, and another). And all of them had caveats and warnings. I felt comfortable doing this to our old computer because a. the solution seemed to make sense (though I'm not an expert) b. all the data is backed up to standalone drive every night, and c. my next step was going to be to reformat the drive and start with a factory system restore anyway…
But given all that… it worked, it worked, it worked!