Working with AutoCAD 2009 the other day, it suddenly complained of a lack of memory. The Vista computer upon which I am running it has 2GB RAM and 92GB free disk space.
Earlier, I found a memory cleaner that frees up memory that's being hogged by applications. When I ran it, it reduced the amount of used RAM from around 1500MB down to the normal 1000MB. After that, AutoCAD was happy again.
I now run Memory Cleaner just about every day after using AutoCAD 2009. You can get a copy of the utility from www.vasileios.gr/freesoft. (Strictly speaking, it is a graphical frontend for Vista's own FreeMem command. Mr Vasileios also has a version for XP.)