I am facing 2 problems.

I am using Internet in LAN (24 Hours connected). using proxy server.
I could not access my yahoo “sign in” page, whenever I entered www.mail.yahoo.com or www.yahoomail.com it say either page could not be open or opens a blank page with any thing.
Some times it opens the sign in page but after taking the user name and password it says “the page could not be open” or gives me a blank page.
I have couple of yahoo accounts but could not open any of them.
I just installed a fresh copy of windows because of this problem but the problem still exists.
I have also updated my copy of windows and also installed service pack 4.I have also deleted cookies and temporary files but no avail.
I am also using Microsoft antispyware.
I have also tried Opera and Firefox.
Firefox sometimes gives error in pop window that “the connection was refused when attempting to contact login.yahoo.com”.
I have also done online virus scan but I am still in problem.
However this problem is not permanent but most of the time I am facing this problem.

The 2nd problem that I am facing is that most of the time CPU usage remains 100%,I don’t know what is the reason some of the process details are as follow

IEXPLORER.EXE sometimes using till 99% of CPU.
SERVICES.EXE sometimes using till 60% of CPU.
realsched.exe sometimes using till 45% of CPU.
And also
System Idle Process sometimes using till 94% of CPU.

Please Help me, I am very much worried because these problems.