There is no reason that vista would lock you out of using the shift key. I would suspect either a hardware flaw in the keyboard (damage) or you are not clicking and holding down the key together.
Lets try this – go into word.
Type some garbage.
Hit control a
does it select? de-select and try again.
If this continues to work I would suggest an anti-spyware or anti virus scan due to the fact that they can slow down the system response time and then the keyboard + mouse actions will not be synced up.
Has you keyboard or mouse sustained any damage?
How long has this problem been going on for?
Have you tried using system restore to go back to a point before that?
From Yu. Vlasov:
I’m sure my keaboard and mouse work properly.
I have no troubles with the other applications – Notepad, Word etc.
I successfully select text by CTRL, SHIFT + mouse click and arroow keys.
Only Explorer works so.
BTB, the same folder on various days allows or does not allow multiple selecting.