I had an odd problem with a Windows Vista SP1 installation. I suddenly began getting "your computer is low on memory" warnings when nothing really had changed with the way I use the machine. I looked at Task Manager and saw that it was showing page file use of ~1700M / ~1800M. When I went to Advanced System Settings (System Properties) to examine the page file size (Advanced tab, Performance settings, then the Advanced tab again) it showed the total page file size as.... 0 MB!
I knew I had enabled a page file previously when I set up the machine, so I rebooted--and it still showed 0 MB for the page file size. So, I went to assign a new 4096 MB page file. When I did this, it gave me a message stating that pagefile.sys already existed (!) and asked if I wanted to overwrite it. I half expected the machine to die painfully when I approved this action, but it did not... and knock on particleboard, I haven't gotten any of the warnings since.