-Try flushing the DNS settings:
http://support.apple.com/kb/HT1527
-Make sure ssl 3.0 and tls 1.0 are enabled in the advanced internet options:
http://support.apple.com/kb/TS1470
-You could also try unchecking "check for server certificate revocation" in the advanced internet options.
-You could also try to check this:
http://support.apple.com/kb/TS1814
-Restart your computer after these steps and then test with a free song or video from the itunes store homepage if possible
-if none of the above steps resolve the issue, it is probably your anti-virus/firewall, web filter, spyware blocker, etc.
-try updating these softwares and if all else fails try a new user account (administrative privileges) or uninstall/reinstall anti-virus software.