The help file says:
Can ArcGIS 10 be installed on the same machine as ArcGIS 9.3.1?
No. For users who want to install ArcGIS 10 and ArcGIS 9.3.1 on the same machine, Esri recommends using a virtualization tool (such as VM Ware's VM Workstation or Microsoft's Virtual PC) and install the new instance of ArcGIS on that virtual machine.
We had initially announced that we would be able to support running ArcGIS 10 and ArcGIS 9.3.1 on the same machine. However, we were not able to accomplish this for ArcGIS 10.
(FWIW, I'm successfully running Hawth's Tools in ArcMap 10. Although it's not listed as being supported at this version, it seems to work.)