In the most simplistic terms, virtual memory is using disk space to extend physical memory size.
Basically what the computer does is it takes a small chunk of your hard drive, and treat it as extra memory, in addition to the RAM chips already in your system.
Paging files is an index table for things that are stored in virtual memory, the bigger the file, the more you can store in virtual memory.
The possible causes are either a particular application of yours is a memory intensive program, or you have too much stuff on your hard drive, or you need to defrag your hard drive to clean it up.
As for the message you were getting "virtual mem too low...", you can either increase it, (there is a settings page where you can do it--more below), or clean up your hard drive, as the previous poster suggested, or find out the application that is using so much memory and remove it.
As for whether it is an issue of concern, that really depends on what worries you. Windows XP will automatically adjust your virtual memory size to accommodate the program's needs. But I usually increase it again anyways so I won't get any more such messages.
It won't crash your computer, wipe away all the data or suddenly teach your computer how to do a jig, but things might run a bit slower.
And could you provide some more detailed information about your system? OS, what programs you are running, RAM size, hard drive size, which service pack you are running, make and model of your machine?
How to increase your virtual memory size?
Follow my instructions at your own risk.
1. right click on my computers, click on properties
2. go to the advanced tab
3. under the performance section, there is a settings button, click it
4. go to the advanced tab
5. you will see three sections: processor scheduling, memory usage, and virtual memory
6. under virtual memory, there is another settings button, click it
7. under the section called "paging file size for selected drive"
8. you will see two fill in boxes that says initial size and maximum size.
As the name implies, initial size is how much of the hard drive is used to serve as memory at the very least, while the maximum is the limit of how much the hard drive is used to serve as memory at most.
Your get your error messages when that maximum limit is breached.
You can also select the system manage size option. I have never tried it.
9. leave the initial box box unchanged, for the maximum box, it has been recommended that you put double of whatever your RAM size is.
I don't know what yours are, so you will have to figure it out yourself.
But keep in mind there is no point to making it too big, law of diminishing returns and all that stuff. But just to give you an idea, my laptop has 40GB hard drive, and I put the maximum size to 1.5GB