<p>Do I risk losing sales by disabling SSL 2.0 and PCT 1.0 in IIS5?</p>

<p>Clarification: Sales would be lost by client not being able to connect via SSL to complete ecommerce transaction because SSL 2.0 or PCT 1.0 is disabled on the web server. </p>

<p>Microsoft kbase article: <a href="http://support.microsoft.com/kb/187498" rel="nofollow noreferrer">http://support.microsoft.com/kb/187498</a></p>

Only you can really answer that question. Your customers' experience of your site will be mediated by their browser. The first place to look for browser information is at a listing of the user-agents that are being used to access your website. Hopefully you have a good log analyzer such as Analog, Weblog, Google Analytics, WebTrends, etc. This is the first place to look and should give you a good idea of the SSL level that your general community supports.
You may also want to alter your application to check for the SSL level supported by your users' browsers that get to the "complete ecommerce transaction" part of your website. This is the best method to determine if you are turning away customers.
Remember that the SSL level is auto negotiated between the server and the client (best encryption used first) so you don't necessarily need to disable older versions, but you could pop up a message to the user encouraging them to upgrade.