where can i download USB-Blaster drivers ?

These drivers are located under the alter instalation path like this example

c:\altera\91\quartus\drivers\usb-blaster

where 91 indicates the quartus version.

or you can use the FDTI driver from their web site.

http://www.ftdichip.com/drivers/vcp.htm (http://www.ftdichip.com/drivers/vcp.htm)

the current version is CDM 2.06.00

under this link you can get the combined driver model D2XX and VCP