Can I use an old desktop screen?
Yes you can!
There are many HDMI to SVGA (which is what computer screens use) converters available on eBay.
I just ordered this one
for my Raspberry Pi. Right now I'm having to use a massive flatscreen... easy to see but not very practical.
I'll post again after I've tried out the cable.
Arise! Arise, riders of Théoden! Spears shall be shaken, shields shall be splintered - a sword day, a red day, ere the sun rises! Ride now! Ride now! Ride! Ride to ruin, and the world's ending! Forth Éorlingas!