My recommendation is to add the lines
127.0.0.1 www.2345.com
127.0.0.1 2345.com
to the existing (essentially empty) "hosts" file, until Mudcat has solved the problem. I would not use the MVPS "hosts" file.

It is possible that the malware only lurks on particular pages of 2345, not on the homepage. It could have been injected by a customer.

Bobad: to make yourself admin if you aren't, you need Harry Potter's wand. Alternatively, log in to a user account with admin privileges (there is always at least one, as treewind writes) when starting Windows. If someone else has the password, ask her/him to do it.

Phil: the Firefox "Back" button may be misbehaving because of the injected <iframe>, or objects injected inside it.

Many reasons to block the 2345 page until further notice.