I already have spyboot, and i just installed windows defender. do the two programs have the same purpose? can i use just one, or having two of them is better? i also have avira antivirus, so I am afraid I am overprotected...?

I am going to assume that you mean Spybot. I run MalwareBytes, Spybot, and SpywareBlaster. I have defender turned off.

Woho, that is a lot. Is spybot itself not enough? and what is your reason for turning of defender?

No single product is 100% foolproof and can detect and remove all threats at any given time. The security community is in a constant state of change as new infections appear. Each vendor has its own definition of what constitutes malware and scanning your computer using different criteria will yield different results. The fact that each program has its own definition files means that some malware may be picked up by one that could be missed by another. Thus, a multi-layered defense using several anti-spyware products (including an effective firewall) to supplement your anti-virus combined with common sense and safe surfing habits provides the most complete protection.

mvps.org is no longer recommending Spybot S&D or Ad-Aware due to poor testing results. See here - (scroll down and read under Freeware Antispyware Products)

Further, most people don't understand Spybot's TeaTimer or how to use it and that feature can cause more problems than it's worth.

I don't trust a company that makes OS's that are flawed to make a decent anti spyware program. The time that I did find it, it never detected anything while MalwareBytes did. I would go without before using Defender.

I agree it is alway better to have more rather than less programs. But when active antivirus/antispyware is concerned it is a bit of a different story, because they consume a lot of memory and slow down the system. I have currently actively running Windowns Defender, Spyboot Teatime, and Avira Antivirus. Each of them consumes more than 50MB RAM and altogether up to 200MB RAM. Then you start to wonder if this is not too much, and if Avira or Spyboot would do fine alone.