I am constantly being attacked each second by an another computer and everytime the attack is from a different IP address. Kaspersky PURE says that is a "Network DoS.Generic.SYNFlood" type attack. 2 Hours back Kaspersky blocked 5 IP addresses and now it blocked 100 IP addresses. What measures should I take ? Am I already infected ? How can I secure myself ?
The only thing that you can really do if someone is trying to execute a denial of service attack on you is what you are already doing (blocking IP addresses). Anything else might mitigate the problem (something like a better way of handling SYN ACK requests) but will not solve it.
Just becuase you are being attacked does not mean you are infected. In fact, since this is a DOS attack the primary goal is to shut you down/make you non-responsive to legitimate traffic, not infect you with a virus (though it is possible to use a DOS attack as a distraction, if the DOS attack is successful, the virus's responsiveness will also be slowed down along with the server).
Blocking 100 IP addresses in and of itself is not a terrible thing, as long as they are actually trying to DOS you. If not, you are just blocking people that want to access your site.