System low on virtual memory

CTRL+ALT+DEL

And click task manager and look at the processes. What proces is using the most RAM?

The problem here isn't due to excessive use of physical memory. It is happening because the commit charge is about to exceed the limit imposed by the current commit limit.
Since you already have 4GB of RAM, the maximum for a 32 bit client OS, there are two potential solutions.
1. Reduce the workload. To check this in Task Manager you must enable the "Virtual Memory Size" column. It will be labeled "VM Size" as the column header. Physical memory usage, labeled "Mem Usage" isn't very meaningful.
2. Increase the initial size of the pagefile. As I know nothing of the current configuration of your system I cannot make any specific recommendations.

If Physical Memory - Total, is about 3GB or more then you likely do not have a problem with RAM.

longshot - how much free space on your c drive? if the drive is full, no matter how much ram you have you can still get virtual memory issues.