Problems with Imagefield

Hi there! I've been messing around with drupal for a few days now - a great CMS overall. However today I ran into a problem and I just can't figure it out.

I am using CCK + imagecache + imagefield for image uploading. I've made a custom content type called my_image - it has a title + an image. Now... when i try do create content I choose path to the image and click "upload" and sometimes the image (thumbnail) doesn't seem to show up - just an empty space with a red "x".However when I submit the content, everything is fine. Now the weirdest part. When the name of the image, which is beeing uploaded, consists of only small letters - everything is fine! Example:

test.jpg -> works fine, thumbnail shows up
Test.jpg -> thumbnalis don't show up
and same is with: TEST.jpg, tEST.jpg, teST.jpg and so on...

Can somebody please tell me what's going on?

I have a local installation of drupal, Windows XP pro on mysql.

Thanks!