Not really enough to go on here mates. 
 
  
Assuming Helo has installed XP and the game flawlessly (which I'm sure he has done), and without tweaking (i.e., disabling) anything yet with the OS, try launching IL-2 with no sound and safe video settings.  If it loads then carefully select each sound/video option one at a time until you run into this problem.
Is it possible that the multiple OS setup is causing a problem?  Didn't you have IL-2 installed under the Windows 2000 OS too?  Can't imagine but is there some sort of conflict?  Just double-check your Registry in Windows XP and make sure your IL-2 entries are valid.
Helo knows all about this PC stuff.  I'm sure you have double-checked all your system components - video & sound card, etc - to make sure everything is working.  New memory is properly setup and recognized, right?  You have virtual memory setup too, right?  IL-2 is probably the largest program you will run on the machine, something like @700+MB plus everything else.  Does it start to load - splash screen with a percentage - then stops?
Let us know how you are doing Helo, and what you have discovered/tried.
Regards,
TS!