Gems installation on Windows

I'm trying to install some gems on my windows machine. Rails, rake, gem
work completely fine. But when for example I'm trying to install
gravatar_image_tag. It installs completely fine but on the windows
command prompt its not being recognized. When I type in
"gravatar_image_tag -v" I get an error message saying that
gravatar_image_tag is not recognized whereas when I type in "rails -v"
its giving me the right output with 3.0.

I tried setting the path to the ruby lib folder in C: but it still is
not working.

Does it have to do with C: permissions? This is really irking me. Any
help would be great.