Computer cannot detect 4g memory card

Hello,

I have a 4gb sandisk ultra memory card, which my computer cannot read. I know it is not a problem with my card-reader because it recognizes my 2gb hi-speed card. I've had to upload all of my photos onto friend's computers for the past couple of weeks, so there also is no issue with a password as far as I can tell. Is there something with the formatting that I can change on my card?? I have a compaq computer that uses vista if that information is also helpful. I appreciate any advice!

Thanks.