You want to be a little careful with limiting the memory because it can have a dramatic effect on performance.
I don't have SQL Express installed (I have SQL Standard).
To limit the memory usage....
Open SQL Server Management Studio (express).
Right click your server name
Click Properties
On the 'Select a page' part, click Memory.
Change the "Maximum server memory" setting.
If you don't have SQL Server Management Studio Express, it's a free download from Microsoft. A quick google search should find it.
-George
"The great things about standards is that there are so many to choose from." - Fortune Cookie Wisdom