If you are running Premiere Elements 9.0/9.0.1 Windows, you have a 32 bit application whether you computer OS is 32 bit or 64 bit. And, as such, it has the liabilities of a 32 bit system, maximum supported installed RAM = 4.0 GB with 3.0 GB or less of that available.
So the questions include
1. What is your project preset?
2. What are the properties of your source media?
3. What do you have on the project's Timeline
a. video, properties and duration
b. photos, pixel dimensions and how many photos
What is the size of the project when you get the Low System Memory message? Does your Timeline include any DV AVI Type 1 files?
Does restarting the computer allow you to advance in the project for a time?
Could you give us more information about your computer processor, clock speed, and such?
Let us start here and then decide what next.