I just found a fix that has worked for many other users with the same 10.5.3.3 issue and it fixed a seperate issue where Apple Mobile Device service drives up CPU by 50-80%.
The fix is to open a command prompt as an Administrator and run c:\netsh winsock reset After running this commad you will need to reboot.
Here is the orginial article:http://www.thebambers.me.uk/wordpress/?p=316
Although my account is the only Windows 7 account and is an Administrator, I still could not run the command above. I first had to open a Command Prompt as an Admistrator:
1) From the main Windows icon (bottom left Windows Flag), select All Programs
2) Then select Accessories
3) Then right mouse click on Command Prompt and select Run as Administrator
4) execute "netsh winsock reset" and reboot
After performing the steps above, iTunes Store is working great and the CPU with iTunes loaded is averaging less than 10%....A MIRACLE!!!!