partitions. Simply learn to install things in the D drive. The C drive was probably only intended to be used for the Boot [Operating System]
For almost all programs if you do a custom install you will get the option as to where you want to install the program. Most offer c: Program Files. so make a Program Files folder on the D drive, so then when offered one can simply delete the C and Make it a D.
You can also move the stuff from the C drive that you don't need/want there. XP's Files and transfer Wizard should make it easy and still retain the necessary links to the .dll's etc.
Frequently the Not enough memory is only telling you that your Temp folders are full and the program doesn't have enough space to put its scratch files in while it is doing an install or update. Empty the Temp folders. Programs are supposed to clean their scratch files out but many fail to do it.
Using partitons have a lot of merit if only people would understand how to use them.
Each partition is treated as if it were a separate drive. Thus you can reformat one without affecting the data on the other. Reformat the boot partition and you need to re establish the dll's by reinstalling the application or?? software, but you do not lose any data.