Can't open Avast or Defender even in safe mode.

I’ve never had any trouble with this before but my other computer is having a problem now, and I’m out of my league here.

everyhtign went kaput when I clicked a link on a site I had been using fine the day before, and now I can’t use my antivirus programs and whatnot.
I can’t open any of them, not Defender, not Avast. It says my Defender is turned off, which it wasn’t.
I cant even run anyhting in safe mode. I cant even open my task manager!
not even as an administrator in safe mode.
Half the time my computer wont even start, just hangs on the welcome screen.

My defender won’t initialize, It tells me to restart to fix the problem, but that doesnt help. then it says I can fix it manualy by doing somethign in the windows defender progam… which I cnat do b/c it wont initialize, and I can’t do a boot time scan of avast, bc I cant open avast…

So thats my problem. I’m Running Windows XP Home. Advice please???

To me this looks like a problem to solve with a Bootable rescue CD, Dr.Web is often recomended in this forum
http://www.freedrweb.com/livecd/

Dr.web user manual ftp://ftp.drweb.com/pub/drweb/livecd/LiveCD-en.pdf

and here is more
http://www.askvg.com/download-free-bootable-rescue-cds-from-kaspersky-bitdefender-avira-f-secure-and-others/

very advanced, Hiren’s BootCD 10.0
http://www.hiren.info/pages/bootcd