Green thing on address bar on vista

[shadowfox]

Hey I am new to vista and everytime I open a drive or folder I see this green thing
flowing through the address bar. Can anyone tell me whats this green thing supposed to be.
theres one thing though, when the green thing is on the adress bar the computer becomes slow. Is this go to do anything with windows defender..

heres the picture of the green thing I am talking about in the attachment

 

[bluedame]

i think it is sort of like a progress bar..

 

[shadowfox]

yes seems like it
but i cant acess my folder if its on the removable device quickly..
takes lot more time then Xp just to browse
however i scanned my p.c for virus there was nothing
also i disabled indexing and windows vista default windows defender but the green thing still exist