First test: move the PC from upstairs downstairs and try there. If it's still slow, it's the machine. If it's fast now, it's the cable.
I recently got my internet upgraded to a 40mb down and 10mb up. Downstairs it works perfectly fine with full connection. However upstairs even when using a wired network adapter I only get a fraction of that (around 5mb down and 1mb up). Any ideas what I can do?