CTD in FS2004

I'm experiencing a CTD with FS2004, but I'm not sure what the issue is. Drivers are current, just downloaded and installed. What's happening is I can start the aircraft's engines and taxi out no problem - external views and cockpit views work fine. I have noticed that after some time with the engine going, the aircraft first goes grey (inside and out, except for the instruments on the panel) then eventually everything freezes. I can't shut down normally but have to do a ctrl-alt-delete instead. The error message I get after shutting down the sim says I have a "hungapp." I've tried deleting some scenery which could be eating up RAM while the sim is running, but am at a loss about what to do next. This problem occurs with the stock B737-400, Milton Shupe's Beech 18s, and any other aircraft I try to use. The text for the error msg says "szApp Name:FS9.exe; sz App Ver: 9.1.0AD901; sz Mod Name: hungapp; szMod Ver: 0.0.0.0; offset: 00000000". I am running MS Windows XP Pro, Version 2002 with SP 3, using an AMD Athlon 64 Processor, 3000+, 1.81 GHz, 2.00GB RAM. DXDiag says I've got no problems with my DirectX or NVIDIA settings. I am also using GameBooster to shut down as much as I can before I run the sim. I'm suspecting a virus of some kind. virus scans and malware scans haven't come up with anything wrong. This is very depressing, being retired (finally) and at last having the time to use the sim much more than in the past, only to have this happen. Is there anyone out there who can tell me what this thing is doing?Thanks