Why have i started having messages about my virtual memory being too low

It means you have too many programs running in memory at the same time. This is usually caused by adware/spyware running in the background. Go to add/remove programs (start->(settings)->Control Panel) and remove 1) any programs you no longer need 2) any "Toolbars" 3) any programs that mention "save" or "shop", as they are adware. Once done, install and run SpyBot:
http://www.safer-networking.org/en/mirrors/index.html
to remove any other spyware. Reboot your computer. Next i would suggest installing and running the CCleaner issues scan (second box down on the left of the program) to resolve any registry issues:
http://www.ccleaner.com/
Scan, then fix all issues, then scan again and fix till you scan and no issues arise. Reboot again.

Have done most of this already so helpful to know I am on the right track. Had not thought of the toolbars. So thanks for that.

Also, turn off unnecessary startup items using mike lin's startup control panel here:
http://computeradvice.mikebinns.net/startupItems.php