I fixed the problem this way ....
After trawlling through some of the forums and microsofts technet I found the following.
Looking at my boot.ini (which i can see in parallels) file you can see a specific kernel and hal file is being used see below
multi(0)disk(0)rdisk(0)partition(3)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect /kernel=ntkopvs_.exe /hal=halopvs_.dll
in this case the hal.dll file is halopvs_.dll Unfortunatly on the install disk there are several Hal files that you could used (depening on if you have a single processor or mutiple processor machine). To find out the on e that is being used open the setup.log file which lists the one that was installed when you installed bootcamp. (eg notepad c:\windows\repair\setup.log)
\WINDOWS\system32\hal.dll = "halmacpi.dll","293a1"
From this in my example you can see that I had installed the halmacpi.dll
So to get the orginal back I do the following
* Insert Windows XP SP 2 disk
* Bring up a cmd window
* run the following comands
cd %TEMP%
expand d:\i386\drivers.cab -f:hal*
copy halmacpi.dll c:\windows\system32\halopvs_.dll
Hopefully that will fix your problem, it did for me except windows does not shut down now in boot camp all the time.
RANT ALERT
I am started to now get very miffed with Parallels as this has happened twice to me now. Also I have started to get Kernel Panics as well, looking at the panic.log was casued by the Parallels Kernel Extension! Plus it has just taken 5-10 minutes to start windows (This is ona 3Ghz Mac Pro with 4GB of memory!). Just even starting a cmd window takes a long time