I had such problem with an out-of-the-box installation of MinGW gcc version 4.5.0 and solved it the following way:
1. Download libmpc-0.8.1-1-mingw32-dll-2.tar.lzma from http://sourceforge.net/projects/mingw/files_beta/MinGW/mpc/mpc-0.8.1-1/
2. Unpack libmpc-2.dll from the downloaded file (for example with 7-Zip) in the bin sub directory located in the MinGW root directory.
Good luck to you on your way, you will need it (as anybody).
If everyone do his/her job properly such problems will be less likely to occur.
Code::Blocks bundled with MinGW does
not use gcc 4.5, but 4.4 and as far as I know it normally works out of the box on Win7.
We get many "bug-reports" of users who mix different MinGW versions on different locations, that might interfer.
The original poster did not tell exactly which version of C::B's installer was used (with or without MinGW) and which version(s) of MinGW are installed and from which sources the version(s) is/are.
Without at least a little willingness of the users, we can not find out, whether the problem is on our side and/or how to fix it.
OpenSource software like C::B (and by the way commercial software also) can only be improved with the help of the users, otherwise errors, that did not occur before (or at least nobody told us about) can not be fixed.