Windows

The most likely culprit is either the Internet proxy settings on the gateway PC or the Internet Connection Firewall settings on the gateway. If either of these is not configured to give the best pass on it will not make the connection.

The best way to rig this network would be to have your ADSL device as the gateway if it is capable of it.

My home network has an Alcatel Speed Touch Pro ADSL Router/Modem with 4 Fast Ethernet ports. One port links to the 100 mbps switch which has seven PCs always linked up (big family OK, bloody live in relos), and capability for another 11 to tie in via the patch panel (whole house is Cat5e wired). Everyone gets Internet acces via the router.

The Alcatel has Network Address Translation capability, some basic firewall aspects, DNS server ability and DHCP server ability. With it also acting as DNS server and Internet gateway every machine can talk to it direct and have good Internet access whilst leaving the actual PCs free to do what we want. It also means that the router maintains the link and no one can take it over to send e-mails etc as there is nothing there to take over. Nor do I have to worry about anyones Internet access when I want to take a PC off line.

If your ADSL connection device is not capable of acting as the gateway, then I suggest you invest in the purchase of one that can. here in Aust you can get basics like this for around A$200 or top line ones with full firewall capability, packet sniffing etc for around A$1,000; suit your own budget and security needs.

Typically when you have a network, and hosts can not access 'some' sites, but can access others, and that network has a gateway machine using PPTP or other such encapsulating functionallity, there will be a MTU problem.
If the MTU on your client machines is set to the default 1500, then when large packets hit the gateway, and the encapsulation is added, you have gone over the MTU limit, this is usually gracefully handled by fragmenting the TCP packet into multiple frames. However if the DF bit is set in the header (Do not Fragment) the packet will not be fragmented. It may also be being droped by another router upstream. Judging from your ping results, the sites are probably blocking all ICMP packets, including those that allow for path MTU discovery.
I would try this test, to see if this is the problem in your site.

Go to one PC on the network, and change the MTU to 1412, and try one of the sites that you have problems reaching. If that works then you have found your problem.