Hmmm, both of those seem ok. 2GB is a little light these days, but if all this computer is doing is a little "Office" work, then it should be enough.
74GB of storage is definitely enough too.
Can you see a pattern in the size of the XLS files that cause problems? As in are the excel files that produce the error over 10MB (or something like that)?
Are there any other scenarios that produce the same or similar errors? (word, internet explorer/firefox, etc)
Do you have more than one hard drive in your computer? Or maybe this happens when you're using an XLS file off of a USB drive? Maybe even a network drive?
As for why I suggested disk clean-up, I suspected that Excel was complaining it was running out of "something" combined with the insinuation in your OP that you couldn't actually save the file when this happened. Therefore, I suspected storage (not memory; memory = RAM).
Just as regular housecleaning, you should run disk clean up and defragment. So if you haven't done this in a long time, go ahead and do it just in case. Could only help.
(within disk clean up, if you're uncertain if something should be deleted, don't delete it, uncheck the box - however anything saying "temporary" should safely be deleted)
In short, I'm a little puzzled based on the information you've given. If you can answer some of my questions or include other information, maybe we can find the culprit. None the less, do the house cleaning, always helps a tad. (fyi, defragment can take ages, so do it when you're not going to be using the computer for an hour or 3.)