dllimport call in aspnet application makes the application hangs

As our application design requires us to use one of the validation API engine
which was written on C(so basically all I have is a dll) and I tested calling
the functions using small windows application using DllImport for each
functions and it works without any problem. But when I try to use the same
Dll in my asp.net web application, it hangs with the first call. I tried
giving the dll path as a hardcoded path like C:\validation.dll in my
dllimport funciton call but did not help.

I am wondering it has something to do with ASPNET account security. So I
gave full control for ASPNET account for my application directory and still
my application hangs.

Anybody faced this earlier?