Looks like a DNS problem?
Most ISPs have their own DNS servers that your connection gos through, and sometimes they are out of date, bad caching or there are 'sites' missing.
You can use other DNS servers, you'll have to ask around or google for a good public one - you can't use the DNS server from a friends ISP as the vast majority (99.9

of ISPs restrict access to their DNS server to only their i.p range.
Once you find an alternative DNS server you just have to enter the 2 new DNS server i.ps into your router config.
Also, your ISP might have access to an alternative DNS server, so trying using the alternative as your primary.
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If it isn't a DNS issue, which it probably isn't since I just read your edit, then you can just use a proxy to access sa-mp.com? If other computers can access it on your connection what about trying another browser on the machine in question, sounds like a security setting or your browser being slighty fucked via spyware/virus - evne if it was removed the removal could have taken something with it - or even a bad cluster on your harddrive which happens to be one which your browser is installed on .... difficult to know