Patents available for website GUIs?

A patent gives you the right to exclude your competitors from practicing your invention. Whether to pursue it depends a great deal on how easy it would be for a competitor to circumvent your specific invention. A web interface would be patentable most likely as a design patent (specific look of your interface) or a process (steps methods of interacting through your interface). It would be difficult to define such an interface as only a "structure" (product patent), without incorporating method or process steps (I've filed over 60 software patents as a firmware engineer, so have some experience in filing interface applications). The issue you face is in defending your patent if you succeed. You need to at least threaten infringement of a competitor (and be able to provide their interface uses at least one of the elements you claim) but more likely need to be able to back that up with a time consuming costly infringement suit, or get them to pay you a licensing fee (which is better option, assuming you have a valuable interface).
Filing a provisional application gives you 12 months to either file a true utility application (a design patent cannot take advantage of a provisional application, if you chose a design patent for the structure of your interface). It only provides protection if you both file and receive a grant for the non-provisional utility application that follows (the costly, 3-4 years). So your decision needs to be based first on how easy it is to get around your invention (low value patent), how hard it is to discover infringement by another (hard = lower value), and how much a competitor would just as soon pay you a reasonable license fee to use your invention, or parts of it (using any part is infringement). If you have a really good invention that someone else would pay to use, then a patent has better value to pursue. In all cases, you should think much more in terms of licensing value to a competitor over exclusion. Exclusion means infringement suits -- where patents are concerned, the carrot (license) is almost always the greater value than the stick (exclusion) unless you have very deep pockets to go after what may be a large number of infringers (the web world having lots of competitors). Hope that helps!