With this issue.
1. move your cursor over the yellow avast symbol bottom right hand side.
2. Left click it
3. Go to AVAST! Shield Controls
4. Disable for 10minutes
5. Try updating steam now.
6. Once update has finished , do the same to re-enable AVAST.
(if it doesnt work, go into your virus chest on avast, and right click on the file vstdlib_s.dll file, and click restore)
Thanks Avo.