(not responding) in Vista

[#1]

Why am I getting (not responding) fairly often these days? What does it mean? 
Regards
Colin

[#2]

What is 'not responding', your browser?

[#3]

It can happen in Chrome, Firefox or any other application. It can even happen in Windows Explorer. At the top of the window next to the name it shows (Not Responding).

[#4]

it is not an error, it is just telling you that because of low memory, the program that you are working with is not responding or is taking long to respond therefore when you click on opened program it gives you that menu

to proceed.therefore:
You either need to increase your memory or paging file or need to run less programs at once. if you do not run many programs then u can fix it by minimizing your startup programs and also make sure that your system is clean.
therefore
perform a scan using
http://www.malwarebytes.org/
http://www.systweak.com/AntiSpyware/
http://www.avast.com/en-gb/free-antivirus-download
make sure your system is clean of any virus, trojan, spyware and malware.
then get rid of junk files using ccleaner
http://www.ccleaner.com/