Hi Igor, I had that casio QV3000EX cameras.
And i loved it.
Too bad I had to sell it to get my G5, but it was worth it.
Yes that camera had UMS (universal mass storage class device)
it worked transparetely as a drive.
Unfortunately Canon droped that USB protocol with G5.
Who knows, maybe for better remote control from desktop or more compatible direct to print features.
Your camera will show up as a drive in winXP.
But due to the PTP (Picture Transfer Protocol), you'll see only image files, CRW and JPEG, and maybe some tiff too.
If you want to use it as your portable drive there is a work around.
Rename the files you want to tranfer with extension CRW or JPG.
That way they will show up.
But you have to rename them back to their original extension.
This might work for a while but is a nuisance.
You can get card readers for $20 or less, and some of them USB2, will be much more faster than the USB 1.1 of G5.