When I try and download

It's very hard to troubleshoot software problems online, and it likely is a software issue. But as a solution I highly recommend that you instead consider a card reader to download your photos to your computer. They're very inexpensive, much faster downloading photos from the camera, do not use the camera's batteries during download, and are much less prone to file corruption of the photos during the download.
You place the camera's card in the reader, and your computer sees it as a hard drive. You simply copy/paste or drag/drop your photos onto your hard drive. For all of these reasons, most professional photographers utilize card readers exclusively. Really, you'll save yourself a lot of heartache. Here are some examples from Amazon. Also, don't be fooled by the cost. some of the cheap ones work just fine. Go by the reviews. Also make sure you get a reader that is stated capable of reading your card (particularly if you have SDHC):

http://www.amazon.com/s/qid=1232260593/ref=sr_st?keywords=sd+card+reader&rs=172282&page=1&rh=n%3A172282%2Ck%3Asd+card+reader&sort=acrelevancerank

For international readers, here's another source for card readers (free international shipping too :-)

http://www.dealextreme.com/products.dx/category.300