I'm assuming here that you haven't updated to WM5? Main memory consists of RAM, and is used for two purposes in WM2003SE (which is the OS on your device). The first is analogous to RAM on your PC - it's where programs are loaded when you run them. It's also used as a storage area, much like a RAMdisk on a PC (if you're old enough to remember that far

). The iPaq File Store is quite different, and is made up of flash ROM - think of it as your PDA's hard drive.
So since you've only have 64MB RAM, and you've used up 62MB, that means you have about 2MB to run programs - the minimum you should have is 10MB. Deleting the preinstalled programs will not help, even if it were possible, as they are stored in the non-user accessible section of the ROM. A few things you can do:
1. Move as many of your files from My Documents to your SD card. You might also want to use a CF card for the same purpose (cheaper than SD and in bigger sizes).
2. Clear your internet cache (launch Pocket IE, go to tools->options->memory tab->delete files).
3. Soft reset once in a while.
4. Shift your less used apps from main memory to your SD card (you'll have to uninstall and reinstall).
HTH