can i connect ADSL modem to switch instead through router?

Answer 1

This all depends on the ADSL modem. If this is an ADSL modem/router which does network address translation than yes you can hook it directly to a switch. If this modem does not have internal routing capabilities then you can hook it directly but only one pc will be allowed outbound at a time. An easy way to do this on a windows machine XP and below is to click on start -> run - > cmd -> and type ipconfig if your IP address is in one of the following ranges it most likely supports nat

192.168.x.x
172.16.x.x
10.x.x.x

This is better answered by your ISP.

Answer 2

No likely not, switch has no programming that dishes out Internet sharely.