If it works on your computer right beside you then you don't need to look any further then your own computer, because then it is a local problem and nothing else.
Check your hosts file if it got the site in it (and thus blocked)...
You'll find the file here:
Windows 95/98/Me => c:\windows\hosts
Windows NT/2000/XP Pro => c:\winnt\system32\drivers\etc\hosts
Windows XP Home => c:\windows\system32\drivers\etc\hosts
(you may need administrator access for Windows NT/2000/XP)
Also see
http://accs-net.com/hosts/what_is_hosts.html
EDIT and further update:
Problem solved, it was indeed his hosts file which blocked the Plus! site.
Note that hosts files taken from sites such as above are updated regulary and if you're using such hosts files it is recommended to update them to the new ones.
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