Can't seem to make a CD that'll work, despite the fact it happily plays CD's I made 5 years ago.

The ones I can make using Win Media player v11, wont work in the car, (but play on the PC or my home CD player OK).
Older ones, (made with Nero Burning Rom, on my previous desktop m/c), play OK everywhere.
Downloaded free trial version of Nero, but that generates an error & spits the disc out everytime I try to burn one.
As far as I can tell, there is no difference in the file system or format of CD's that play, or won't, but obviosly something's different / missing as the car's player loads them, doesn't find anything it understands and moves on to the next one.
Any clues, people?