User Account Control. One of Microsoft's brilliant security ideas.
help for it can be found in the Windows Help and it's switched on/off in a section of the Windows control panel (on by default).
probably one of the reasons I've been fortunate is that I plotted the route ahead of time
1. reconfigured the base drive, adding a twin and setting up RAID
2. installed the OS without all the "extra" bits, toys and tools
3. configured the drivers and made sure they worked
4. installed a new, clean 3rd drive to hold FS - it doesn't have to be huge but should be fast.
5. installed FS before any other programs, configured the drivers to fit and tested for a week before any other steps.
6. installed a browser (FF) and have rigorously maintained a clean system
7. after 3 months installed Office and a few other bits to make life easier.
-- oh, and you should make regular backups along the way... I use Acronis True Image to make an image of the drive before every change.
Being careful and planning takes a bit more time before getting FS running but it's been a blessing in the long run.