There are official motorcycle parking spaces in River Street and Alexandra Gardens car parks. There's a good map of all parking areas here - www.thamesweb.co.uk/parking
You can also get away (at least currently - 2006) with parking on the pavement just past the car park turning on Charles Street. There is some cycle parking there too.
Windsor MAG is proud to say that we applied some pressure to Slough council to provide dedicated secure motorcycle parking in Slough. This proved to be essential after they privatised the traffic wardens and enforced a no tolerance approach to parking.
Slough council has a page with the locations - www.slough.gov.uk/services/1932.asp
There is also free motorcycle parking in the Observatory car park. Just follow the signs. (Use the right hand lane on the way in).