Crash to desktop.....

Hello Fellow simmers, My PC/FS9 has developed a nasty habit of crashing during flight. I've played around with various settings and scenarios and even deleted anything to do with FS9 and reinstalled. The problem remains. But it only seems to happen when I'm flying an add-on aircraft. If I fly using the default a/c = no problem, but soon as I load up either a payware a/c (PMDG 737 6/700 for instance) or a freeware a/c (from the Avsim library, of course..where else??) within a period that could be seconds and sometimes minutes the screen goes blank and I'm left reading the "Microsoft Flightsim' had to close....." or it cuts straight to my desktop without so much as an excuse me! In the winter of FS9's life I'm not too concerned as I'm busy enough enjoying the summer here in Scotland(such as it is) and can wait for FS10 (whenever that maybe!) for hardcore sim time. Once we know which way the wind is blowing regarding hardware requirements for FS10, I will be buying a new PC. I was just curious as to why this might be happeneing. Maybe someone else has experienced and resolved this?....I have a registered version of FSUIPC, the aforementioned PMDG, FlyTampa Dubai and that's it for add-ons. Pentium IV (2.4ghz), ECS 648FX-A MoBo, 1024MB SDRAM, NVidia GeForce FX 5700, Maxtor 5T06H6 60gig HDD, Windows XP SP2 and regualrly update all my drivers. Thanks for reading/commenting and regards to all in this great community.Stan Harvey (EGPE)