We're having the same problem from both Windows computers in our house. The iTunes network troubleshooter (Help > Run diagnostics) says:
Network interfaces verified.
Internet connection verified.
Secure link to iTunes Store failed.
ISP is Bell in the Toronto area. Is anyone else having these problems? Is something wrong on the Apple server side?
Microsoft Windows 7 x64 Home Premium Edition Service Pack 1 (Build 7601)
iTunes 10.6.1.7
QuickTime 7.7.1
FairPlay 1.14.37
Apple Application Support 2.1.7
iPod Updater Library 10.0d2
CD Driver 2.2.0.1
CD Driver DLL 2.1.1.1
Apple Mobile Device 5.1.1.4
Apple Mobile Device Driver 1.59.0.0
Bonjour 3.0.0.10 (333.10)
Gracenote SDK 1.9.5.502
Gracenote MusicID 1.9.5.115
Gracenote Submit 1.9.5.143
Gracenote DSP 1.9.5.45
DNS Servers:
Active Connection: LAN Connection
Connected: Yes
Online: Yes
Using Modem: No
Using LAN: Yes
Using Proxy: No
Firewall Information
Windows Firewall is on.
iTunes is NOT enabled in Windows Firewall.
Connection attempt to Apple web site was successful.
Connection attempt to browsing iTunes Store was successful.
Connection attempt to purchasing from iTunes Store was unsuccessful.
The network connection timed out.
Connection attempt to iPhone activation server was successful.
Connection attempt to firmware update server was successful.
Connection attempt to Gracenote server was successful.
Last successful iTunes Store access was 2012-04-20 20:30:52.