I had the same loud sreech before the window mech cable snapped. It's only half hour job to take the door card off and get at the mech to lubricate the cable and runners which could save you £200 for a new mech, may only delay the inevitable but worth a go. I can explain how to get the card off (Laguna II) of you need the info.
PS If you do the job make a note of the type of window mech you have (2 or 6 pin type the latter is fitted to cars with the comfort mode i.e. anti pinch mech, these can be obtained off e bay for around £110 and fitted in minutes.