[Shub-Nigurrath]

dear ivanov,

you haven't told if it's a native VB app or a p-code one, anyway changing VB apps isn't anything different than normal applications, the only difference is that there's a more frequent access to the VB runtime dlls, which complicates to follow the program's flow.
Generally speaking the only things you shouldn't change (up to you don't know what are you doing) are the jmps tables, which are used to find message handler in the program..

The behaviour you told can be due to some crc-like checks, try to see with Peid and the Karnal plug if there's one..