If your OS is Vista or XP that might be the biggest problem as MS may try to tell you that its a different PC and you need a new licence.
Apart from that, just putting your old hard disk in and booting is asking for bother. It may work it may not but you will have problems sooner or later. There are a lot more chipsets on a mobo apart from the N and S bridge.
Best to do a format and a fresh install.