Do you have a central heating radiator that has any unpainted parts visible? While it may not necessarily be grounded, that sheer mass of the central heating system would present a low resistance destination for any residual static you may be carrying.

Also... your computer would be grounded, so touch any metal parts of the computer case. Unless it's painted with something insulating, it would do a similar job. One of the chassis screws on your PSU would be a good choice.