Error 0x8007043C

I am having problems with my PC (Windows XP) and have run all my scans etc to
check for viruses and it has found nothing. (Ad-aware, Symantec,
Spywareblaster also cleared out cookies and tif's, done PC disk clean up and
defragged).

My PC will start up but is then dead and I can't do anything. I can't even
shut down - I have to switch it off manually (and even to do that I have to
press and hold the button for about 10 seconds before it actually shuts
down!!). I have left this msg whilst in safe mode.

When I try to update Windows in safe mode I get this 0x8007043C error code.
Have read all the questions and answers on here and the reply is always the
same - you can't do it in safe mode.

When that's the only way I can do anything at all with my PC, how else can I
sort this problem pls???

If updating Windows won't help to sort out my PC, any other ideas other than
sending it away to get it fixed??

All help greatly appreciated.