NT4 (SP6a) Freezes on Boot

Hi,

I have a Windows NT 4 Workstation machine, with a freshly installed SP6a. However, since installing the SP the machine regularly (but not always) freezes on the blue boot up screen.

Reapplying SP6a has not helped.

The hard disk is 10Gb (6Gb NTFS drive C, and 4Gb NTFS drive D).

Any ideas?
Thanks.