I can’t access any .se domains!

Dear All,

I have an interesting problem with my computer lately and would appreciate any help in order to resolve it.

I can't access any .se domains (apparently only them!). These are domains related to Swedish websites.
e.g. https://si.se/

I am not a computer novice (I think!) and have checked my firewall settings but they seem OK (I used to have the ESSET suite).

The funny thing is that I can access the .se domains in my other computer connected wirelessly to the same router! (Although sometimes I need to refresh the pages a couple of times in order to load them, but that can be due to the wireless network quality).

I have now installed the AVG antivirus in both computers instead of the ESSET suite just in case but the situation remains: both are connected to the same router but one can't access the .se sites! So it can't be any router settings or fault.

What else could it be? One computer uses Vista (the problematic one) while the other uses XP. One computer is connected to the router via a cable (the problematic one) while the other via the available wireless network.

In general, both have the same software installed.
I have been using www.vtunnel.com to access the .se domains but can't access all of them and it's not very convenient.

I contacted my Internet Service Provider but they couldn't help me! From what I've read online so far, some people say this could be a DNS related issue. However, I haven't found how to solve this (I have also flushed my DNS and restarted my DNS Client).

Apart from that, my internet connection seems flawless.

I would welcome any help and really appreciate your time reading this.

Many thanks,
Alex