Display Problems

Hey All,
Ok, so I've been using AutoCAD 2004 for ages without having this problem,
but yesterday for some reason that escapes me it started. The problem is
this: When moving, copying, mirroring or anything which involves viewing the
objects you are modifying, they disappear. The original line/s stay solid,
but you are also supposed to see the same objects moving across your screen
when you move the crosshairs after selecting the basepoint, all this has
gone, all I see is the original line/s and the crosshair but no dotted
line/s representing the selected objects. This is really pissing me of,
especially when drawing building elevations.
If anyone can help me solve this problem or let me know if there is a system
variable that I can use I will appreciate it greatly. I just don't get why
this has started, I have changed nothing on my system for ages.
Thanks
Mark