dll file doesn't work after upgrade to Windows2000

Hi,

I've just upgraded my system to Windows2000, afterward I reinstiall my
Visual Studio, etc.
The project I'm working on uses one dll file. Now it sometimes give me
run-time errors when I try to call functions from this dll file. But it
worked fine in Windows98.
Can anyone tell me why? Does it mean the dll cannot be use under
Windows2000?

I'm very appreciate if someone can help me!
Thanks a lot!