This says that the RRDTool library is not found in the standard Perl library path, indicating that you haven't installed it correctly.
However, as it also states, this might not be an issue if you have explicitly given its location in the LibAdd: directives in the MRTG cfg files.
I would suggest you obtain and install the latest RRDTool perl libraries for Windows from here
http://oss.oetiker.ch/rrdtool/pub/?M=DThe one you want is the -win32- file (assuming you dont have 64bit windows) and either the Perl 5.8 or the perl 5.10 (depending on which perl you have installed).
If you're using the Bundle, then it is probably the 5.8 rrdtool-1.2.15-win32-perl58.zip
You can also get v1.2.23 form the GKNW repository with:
ppm rep add "GKNW PPM Repo"
http://www.gknw.net/development/perl/MSWin32-x86/5.8/ and then use ppm to install it:
ppm install RRDs