RAM is the component that allows for your speed on your computer. When you have too many things running at once, then the RAM gets full, and will overload. Since you are running on 256MB of RAM, then when you run some of these programs then your RAM will fill up. Then instead of freezing, it will access some of your hardrive space for help (a.k.a. virtual memory). With virtual memory, the computer can use hard disk space RAM. The computer uses virtual memory to augment the ordinary RAM that is installed on the computer. If you already have a low value set for your virtual memory, then it will give you this message to expand it. But in the process some of the programs you are currently using might not be able to work (like your antivirus updates). If you have plenty of hard drive space, then just increase you virtual memory by following these instructions.
1. Click Start, right-click My Computer, and then click Properties.
2. In the System Properties dialog box, click the Advanced tab.
3. In the Performance pane, click Settings.
4. In the Performance Options dialog box, click the Advanced tab.
5. In the Virtual memory pane, click Change.
6. Change the Initial size value and the Maximum size value to a higher value, click Set, and then click OK.
7. Click OK to close the Performance Options dialog box, and then click OK to close the System Properties dialog box.
If this doesn't help, then I would recommend two things...
1. Try not running as many programs at once.
2. Buy more RAM sticks and install them (unfortunately with older computers, which this can be even a year old in today's world of electronics, programs are getting more elaborate and will take more RAM anyways to run, so it is inevitable)