This started before Christmas.
My computer runs from the moment I get up until I go to bed. Most of the morning is spent doing housework or shopping. Once these are out of the way I'll check my emails and my RSS news feeds. I may spend sometime working on the computer writing short stories or looking at or uploading photos on Flickr and a whole host of other things such as working on Photoshop for example.
For most of the time my computer although switched is more or less dormant, with only my browser running so I can check my emails or the news feeds. I may occasionally do some work on a story I'm working on or spend sometime using Photoshop.
It was around Christmas that I started getting a message on my monitor screen, which said "Your computer is running low on memory, save these programs..." At first I thought nothing of it, on the safe side I saved whatever work I was doing and then I restarted the PC. The following night at around 19:40 the same message appeared so I did the same as the night before, The following night I had it again and I've been getting it ever since at exactly the same time. Even when the only program I'm using is my browser.
My memory stats:
3072 Megabytes Usable Installed Memory
Slot 'DIMM0' has 1024 MB
Slot 'DIMM1' has 2048 MB