Windsor is an older town, and like many places in the U.K. does not have an over abundance of parking.

 However there are private and public car parks in Windsor. 

Some of the local privately owned parking lots are notorious for issuing a fine if you're even a minute late past your paid parking time. Former practices included clamping, which has subsequently been banned in England. 

However, Windsor has several public car parks (you will still pay here) where you can park for a reasonable price.  Remember to bring plenty of change--£1 coins are especially handy.

 The following link shows the public car parks maintained by the Royal Borough of Windsor and Maidenhead.  Click on the link to be taken to a map of Windsor. 

If you click on the name of the car park underneath the picture, it will take you to a more detailed view of where that car park is, and the prices.