Then you must learn assembler. But why do U need it?

hey I lost my source code

No, it is not possible to decompile back to C. The best you can do is get the assembly code. Since you lost the source code (and that happened to me a couple times too!) you will just have to rewrite it. That has some advantages because the second write ususlly makes a better program anyway because you already know most of the algorithms needed and you may be able to make them more efficient.

It's possible, but very hard and the result won't be good at all. To decompile you need to disassemble into assembly and translate the assembly into C. If you've seen a disassembly, even a good one, you'll know how useless a translation into C would be. There's just too much lost between the original C source and the object code after compilation. You can get better results from reverse engineering the program based on usability.

dude! this is highly illegal!
more over- this action is against the agreement when you've installed ms products. you can be easily get litigate for this. it doesn't matter if this was originally your source code. (nor if the developing-language is an open source one cause the GPL agreement also forbid Reverse Engineering)

Not only did you bump a long-dead thread, everything you said is way off base.
http://en.wikipedia.org/wiki/Reverse_engineering is specifically allowed in some cases, say interoperability. You wouldn't have a vast choice of PCs nowadays if someone hadn't RE'd the original IBM BIOS to produce a clone.

Likewise (for those with short memories), some non-Y2K code had to be RE'd in order to be fixed.

> it doesn't matter if this was originally your source code.
This of course is rubbish. It was your code, so what's the problem?

> nor if the developing-language is an open source one cause the GPL agreement also forbid Reverse Engineering
And this makes no sense whatsoever. If it's GPL, you have the source anyway, therefore there is no need to reverse engineer anything.
But if you want to try just for fun, I doubt anyone would stop you from trying.