When you say XP reads them fine, are you using the same card reader on XP as on Win 7? As pebbly said, your reader will need to support SDHC to read cards greater 2G. If you are using the same reader and it works on XP but not Win 7 then I would go to the mfgr. website to see if there is a driver
for Win 7 or Vista. Also, are you running 32 or 64 bit?