Try turn off Hardware acceleration for display adapter
We just got a batch of Dell OptiPlex 755 computers a couple of weeks ago. I dropped our standard XP + SP2 image into them and put them into production. All's well until yesterday a user called me asking why he couldn't connect through RDP.
I started investigating and found out that none of these 755s can load RDPDD.DLL. Using the same OS image on another computer model, RDP works fine, no problem at all, so the problem must be model-specific.
The display adapter that comes with these 755s is RADEON HD 2400 XT, which none of our other computer models use. After trying a few tricks, I found the solution:
Display Properties --> Settings --> Advanced --> Troubleshoot, turn off "Hardware acceleration", and off you go.
I know that the solution would apply to similar cases, regardless of the brand of your computer or display adapter, but I do hope that the guys at ATI + Dell could come up with an updated driver soon.
Regards, Wei