It won't run without glitches unless the two computers are identical in every way. The hardware device profiles are a part of the Windows registry, so when you move the HD to the different computer, the registry is going to be looking for the hardware devices in its registry thare are a part of the first computer,when it boots up, as well as when you actually use the computer. So if you are able to get it to boot up, it will not run smoothly all the time.
Second issue is your Windows license. The mac address is a part of the activation of Windows. The mac address is part of the NIC which is part of your motherboard. So when you change computers, you're changing NIC, hence your license will recognize this change, and you'll start getting messages that your windows isn't a legal copy and that it needs activating. This MAY not happen IF you have a retail copy of Windows.....but if it's OEM, it will most likely start right away. You can't transfer your OEM license from one computer to another, but you can transfer the retail copy. So if you have a retail copy of Win 7, and you get this activation message, all you have to do is call Microsoft so that they can change the registration information on their activation server to the current computer configuration.
If it were me, I'd just buy a copy of Win 7. I wouldn't want a computer running glitchy, but that's just me. Win 7 is less than 100.00 if you buy an OEM copy - check out amazon.com and newegg.com for good prices and go from there. They are certainly not the only vendors but they are the cheapest LEGAL vendors (don't get sucked into one of the 'too good to be true" scam sites selling Win 7)