Had to contact Microsoft to get them to re-activate but that wasn't a problem.
Tried the background slideshow and it worked!
So, just in case, I made regular system restore points as I was installing my regularly used software. All was well until I used Tuneup Utilities 2013....
For those that don't know, there is an option to optimize the registry. I do this as a part of my routine when installing this piece of software. Tuneup's optimization stops the background picture slideshow from working! I tried a restore to see if that put the issue right but unfortunately the restore points (I found) do not save the registry, so when I tried to use the slideshow it didn't work.
Luckily I did a system backup after installing my systems drivers and a few updates from Microsoft but nothing else and hey presto, it works again!
Would love to know which registry settings Tuneup changes or removes during optimization though so I can try to avoid this happening again.
Computer type PC/Desktop System Manufacturer/Model Number Acer Aspire Z3771 All-in-One OS Windows 7 Home Premium x64 CPU Intel i3-2120 3.30Ghz 3300Mhz 2 Cores 4 Logical Processors Motherboard Acer Aspire Z3771 1.06 Memory 4.00 GB Graphics Card Intel HD Graphics (built in to motherboard)
Hard Drives NTFS 915.41 GB Fixed Disk
2 other partitions used for system restore Antivirus Microsoft Essentials Browser Firefox 24.0