Reverse engineer C++ DLL

You might also want to have a look at OllyDbg which is a 32-bit assembler level analysing debugger. It is used to analyze binary code in scenarios where you do not have a source code. It is light weight debugger. OllyDbg is a shareware so you can download & use it for free..!!

Visit OllyDbg is home page here

PS: Back in the day crackers used SoftICE from NuMega for debugging into an executable & grab a snapshot at the values of registers. SoftICE was an advanced debugger. It was definitely the favorite tool for the crackers. I don't know about the present status of the product. NuMega's site had no information about it. I may have overlooked it but I could not find it. I recommend that you get your hands on a legacy version (4.0x) of SoftICE & apply the WindowsXP patch for SoftICE. Working with SoftICE is something of an "experience".

Further Read: Reversing: Secrets of Reverse Engineering by Eldad Eilam

Hex-Rays decompiler is a great tool, but the code will be quite hard to read and you will have to spend a lot of time to reverse engineer the whole DLL.