I did indeed use the installer - straightforward and appeared to complete successfully.
NVRAM may well be an issue - it's dead, but there's been no noticeable side-effects other than the clock being incorrect. As you say, it may well be the cause of this, though!
I tried a bit of basic troubleshooting (well, ok, stabbing wildly in the dark). I renamed the various .INF files on the root of C:\ without joy, as well as commenting out ASSIGN.SYS lines to various degrees. No luck!
I'll get the NVRAM replaced and see what happens. Thanks for the reply!