Repair hard disk errors

Hello, today when I turned on my PC a message popped up from the Action Center and said that it needs to check hard disk for errors. I had some problems with computer recently, like freezing and shutting down suddenly. ( I figured out that it was problem with the RAM, so I took out 1GB of ram and that seems to repair the freezing and shutting down.) I want to do that disk error check. So what is it going to do with hard disk, what is it going to check? How long does it take to complete checkind and repairing? Thank you in advance.
System specs:
Windows7 ultimate 32bit
3GB of ram ( it was working on 4GB but i took one out)
HDD wdc wd1600aajs ata device

whs said:

It is probably going to do a chkdsk at the next reboot. But you can do that yourself from command prompt. It will take a little while and depends on the size of the disk/partition it checks.

I tried to do chkdsk from command prompt but it does not complete the check. In cmd I got "Errors found. CHKDSK cannot continue in read-only mode." What does that mean?