By HDD bottlenecked I simply meant that the HDD's reading capabilities were on their limit. When you scan / defrag it takes quite a toll on the HDD since it has to read / re arrange every file. In short, you could use the analogy of a processor being nearly 100% used here and any more load slowing down both of the works it is doing.
Normal browsing on modern computers with enough memory should be fine since most if it is in the memory and can be readily used.
I'm not talking HDD interface here [SATA etc.], infact teh actual drives. Which is why people are pushing for SSD's...
When you load a game it loads quite a lot of data from HDD to memory so that is likely to suffer if there is too much data. Also if its a heavy game and the memory is not enough to load all data at once, HDD activity generally kicks up so in a Defrag / Scan environment that is going to suffer as well.
Hope that cleared it up, let me know if not.
Kshitiz