As an ex-Microsoftie, I'm really happy Skype is doing this. Instant messages are one of the best ways to spread bad things to other people's computers by getting them to click on things. This is going to protect a lot of people.
Google goes through every Gmail that's sent or received, looking for keywords so they can target Gmail users with paid ads. And there's no way to opt out of this invasion of your privacy. Outlook.com is different—we don't go through your email to sell ads.
I've recently discovered that MSN censors IMs with some urls. From reports, it seems that the censor rules are really random too: e.g. everything with the .io tld. It also gave zero feedback that it was doing this, other than "error sending message".
That is not acceptable. If you think there is a virus in a URL, attach a warning as web browsers do with domains that have had reports of distributing malware. And beyond that, actually make sure that you only do this for sites that are really a danger, rather than making up arbitrary rules with exceedingly low precision.
I would prefer they put their efforts into making a secure operating system in the first place instead of trying to nanny their users in such a creepy fashion.
I suspect different teams are involved with scanning for malware URLs in Skype and implementing core OS protections, so the tradeoff you imply is illusory. Besides, Microsoft already puts a lot of effort into the security of Windows, with seemingly good results. There's nothing wrong with defense in depth.
