Double glazing – cost-effective replacements

We're thinking to replace some of quite old double-glazed windows - any suggestions on how to go about this in the most cost-effective way?

Hi Willows ,
Are you going to replace the whole frame or just the unit ? our windows have been in for around 13 years and came with a 10 year guarantee .Slowly but surely over the last 2 years most of the units have blown n need replacing .Beginning to feel as if the window man is a relative

Not sure what's the best bet there Ladynett - I think it's probably easier with the frames. My OH just mentioned you can actually buy replacement windows in places like Wickes - now if he can just convince me he knows how to put these in properly LOL