For your virus scan use a antivirus program, you should have one already. its extremely important that you do! If not try one of these free ones.
All the software listed below is free.
AVG free
http://free.avg.com/
Avast anti virus
http://www.avast.com/eng/download-avast-home.html
For malware:
Spybot search and destroy
http://www.safer-networking.org/en/download/index.html
Malwarebytes
http://malwarebytes.org/
Disk defragment, disk cleanup:
Right click on your hard drive in my computer, select properties, tools and you will find these applications there.
I dont know any free registry cleaners im afraid but im sure you will find one if you use google.
Also a program like cc cleaner will do a lot of these things for you.
cc cleaner
http://www.ccleaner.com/
To find out what is running at your startup:
Click start and type "MSCONFIG" in to the run/search box, without the ""
A new window called system configuration will appear.
Go to the general tab and select " selective startup"
Go to the startup tab and look at the list of startup programs CAREFULLY, You need things like antivirus and microsoft stuff to start up, You dont need stuff like msn, skype or what ever.. you will know what you do and dont want to start best, check the boxs accordingly.
Good luck