Yes, I don't have a problem starting up without the CD drive plugged in.
In fact I've done some more investigating since my post and have realised that it's actually a new harddrive I installed which is causing the problems! (Sorry, I installed the HD days ago so hadn't considered it as a possible cause!)
With my old HD plugged in, both my old CD-ROM and my new CDRW work fine.
I've spent a while fiddling about in the BIOS and changing jumpers around and stuff but I've pretty much run out of ideas now