As mentioned above, you will most likely be in driver conflict hell unless you reinstall the operating system. Two other things to consider:
- Windows licenses are tied to the motherboard, so if you had an OEM copy on your previous machine, technically you are supposed to pay for a new Windows license on the new machine. Windows will probably ask you to update your license key when it sees it's in a new system, and from there I have seen some people say they've gotten it to work with the old license key and some not.
- Depending on how old your previous system was, it may be a good idea to replace the hard drive anyway. If the system was 5 years old, for example, I probably would not use it as a main drive or trust it to store anything important without a backup.