You don't exactly say what the tech level of your customers are but I'd suggest:
1. First tell them it is a great question. Explain to them that your company is very serious about security and they should always feel comfortable asking any question about your architecture, methods,etc..
2. Explain one of the reasons you use Linux is because of your concerns about their security.
3. Be able to link/show them the percentage of infected windows computers compared to Linux. This link should be from a highly reputable news source. (e.g. http://www.nytimes.com/2005/08/17/technology/17virus.htmll
) This is the only stat they need to see.
4. Avoid any evangelism about open source. Most likely they don't care, they want a solution and a provider they can trust.
5. Finally take this as an opportunity to build a better relationship with your customer. The fact that they called you rather than switching providers means they *want* to trust you. Leave them with the feeling that they can.