Some Machines not loading CSS

I'm just learning, so it may be something very obvious, BUT

I've constructured a rather simple web page using some floats
for columns, a Spry horizontal menu and two simple graphic images.

Some computers here at school will not show any images, or
any of the ''column'' formatting.

At first I thought it was IE7, but today a co-worker showed
me his page in FF - the Spry menu worked, but no images and no
formatting.

Other machines - running the same software - show the page
just fine.

We run a mix of Vista and XP machines; all the IE browsers
are version 7 though.

Any suggestions?

Thanks!

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