I've always downloaded my Santander statement in .xls format to Linux Mint 13/LibreOffice/Calc without any problem. This months attempt has resulted LibreOffice defaulting to Writer and presenting the statement in HTML source code. I switched to my other machine WinXP Pro/Firefox into MS Excel and get the message "The file you are trying to open is a different format than specified by the file extension". I get the same result using IE8 and WinXP/Excel. I thought this was a Santander problem (currently re-working their web site), but have just tried to download my TalkTalk phone account with exactly the same result. Other files from these sites (PDF, TXT) download correctly.
That's what I can't understand. I've not altered any of my AV stuff, Linux will open the file but in HTML and this is the first time over three years I've had a problem? The report from Excel regarding the wrong extension made me think originally of wrongly written software by Santander, but now two sites don't work.
Check your router's security settings. I've no idea of your modem/router so have a google how secure it is. Change settings on it to see if that works. Follows you'll need to change connection data on all machines you have. Apparently even routers are vulnerable to being hacked. Failing that then talk to your ISP about the problem.
Odd that it occurs on different systems too, Linux and XP. Browser possibly rules out because of use of Firefox. Router's have hard firewalls but not anti-virus/malware. Wonder if the files are xlsx and not xls?