You can improve the performance of old windows without going to the expense of double-glazing. A simple, cheap alternative is to fit secondary glazing, which can be an additional window fitted on the inside of the existing frame, or a magnetic or adhesive pane to fit to the frame, or even just clear plastic film stuck to the interior frame.
Clear plastic film (polythene) should be available from DIY stores. Attach it round the edge with double-sided sticky tape and then heat it to make it first stretch and then contract back as it cools - to clear the wrinkles. It won't last a huge length of time - probably one winter - but is very cheap.
Rigid (or slightly flexible) plastic sheets are also usually available from DIY outlets. These can be stuck on, or held magnetically (to enable easier cleaning). More advanced options could be a single pane in an aluminium frame attached on hinges or sliding runners, or perhaps an old window cleaned up and fitted inside the frame.
There are also specialist window films that improve the insulating properties of glass by letting through certain wavelengths of light in only one direction.
The most advanced type of secondary glazing is to fit a new second window inside the existing one. This will often be better and cheaper than replacing windows altogether. You could hire a local joiner to make them, or proprietary systems are available - for example from DIY stores.
For advice on choosing replacement windows see Should I get double glazing?