When Windows is running low on physical memory, it uses part of the machine's hard disk as virtual memory. The message you're seeing is telling you that the area on the hard disk which Windows needs is getting below a certain threshold.
As SB says, adding extra physical memory to the machine will fix it. If the memory in the machine is already at capacity, then you have little choice but to buy a bigger machine...
Your installed memory is nearly full, I would advise that you try to defragment your hard disk, and remove any old files/folders you no longer need/want. This should releave the occurance of this message, but you really should buy more memory for your system.
Perose,
Apart from buying an external hard drive (I've done so myself ) & clearing loads of your NOW content which you'd backed up externally on that, then yes you should do either of the following:
Install & quickly use CCcleaner (accessible freely by downloading the Yahoo toolbar). For this do a search for ccleaner on cnet downloads
That's free. Then & only then, only use a defragmenter - you didn't say which Windows operating system you use!
But if you you're XP or later you should be alright!
Do it.