I've installed Magento 1.5 on WAMP (at testdomain.localhost). I also installed the sample data.

I am using the default theme and have made no changes to it. 

When I add one of the sample products to the cart and then click My Cart no items appear in the cart (even though in the menu it says there are items in it). 

I tracked it down to app/design/frontend/base/default/template/checkout/cart/item/default.phtml. Nothing renders after the call to $this->getProductThumbnail()->resize(75); 

It seems $this->getProductThumbnail() doesn't return anything but I can't figure out anything beyond that (noob and pulling my hair out..) 

Any help appreciated!

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-- THANK YOU matthew & clockworkgeek, it was the gd2 extension.. I had disabled it while debugging a WAMP issue. Doh!

Also increased memory\_limit for good measure. Everything working perfectly now.