Fitting your own UPvc windows?

[Badvoc]

Have just found out that you can buy UPvc units yourself.
We need to replace 4 windows on the back of the house as cheaply as possible.
Did look into just having the glass replaced but the seals which are attached to the frames are rotten too, so that's not an option.
Is it something you can fit yourself? Are windows easy to take out? Or is at a bad idea?