Patrick,
Let me guess that if you're in an assembly if you try closing just the part that has the error that you still won't be able to save. Try making that part the displayed part and you probably can't.
Does it happen to other boxes or just yours? Are you running XP-Pro, or something else? Does the box under properties report all the memory it was advertised with?
It does sound like your box is coming up against a hard memory limit. It could be that some of the memory has failed, but that probably isn't the case given that we've seen what sounds like the same thing. I've listed a couple of things that will help free up some memory.
Go into the task manager and look at what's using lots of memory if there are processes or other applications that you can afford to be without using lots of memory then stop running them.
Go into system properties Advanced tab, under performance options using the visual effects tabs set it to Adjust for best performance. I don't do this, and it is probably only a 1% improvement, but every bit could help.
In the same area system properties Advanced, use the performance tab, then advanced tab and then change the virtual memory to make the custom size equal the maximum size. If you needed to increase it then you may get some benefit. Also make sure when doing this that you have enough free space on you C drive.
For XP-Pro with 4Mb of memory try turning on the 3Gb switch in your config.sys, we've posted how previously but you'll get instructions quickly via google.
You may not see dramatic results, so save often, and be prepared to reboot at least daily is advisable.
Best Regards
Hudson