I hope this thread is OK to post.
I spent all evening trying to figure out how to get my files from my 256mb Micro-SD card to my new 4GB SDHC card(didn't realize it was HC until I got it or how big of a deal it was).
I couldn't get XP to recognize it because I don't have an SDHC reader.
Well after digging around I found a solution and wanted to post it in here to hopefully save someone else from the same trouble I had.

What I did was moved all of my files (Homebrew, hackmii, etc) to a USB thumb drive.
I also downloaded WiiXplorer and put that on the thumb drive as well.
After doing that I plugged in just the thumb drive and launched the HomeBrew browser.
I put in my new SDHC card and opened WiiXplorer.
Once there I copied each file and folder from the thumb drive to the SDHC card.
Viola done!
I really hope this helps someone!