newbie, hi, techmonkey is right about closing every program and application except systray and explorer before performing Defragmentation.
Now, about installing or using more than 1 anti-virus softwares at the same time. With some anti-virus progs you can use at the same time, and with others, you can't. For example, using QUICK HEALER with any anti-virus progs will not work. QH is notorious for constant re-booting if use with any anti-virus progs on the market. Panda and RAV sometimes do this too.
On the other hand, AVAST! Seems to work well with most other anti-virus progs, except with QH and the like. AVG sometimes work with other progs. And Norton works and McAfee are OK, but NORTON hogs up a lot of memory, so in that sense, Norton may interfere with other anti-virus progs. McAfee, well, I don't think McAfee is all that great, especially their latest version because its internet update doesn't work half the time.
But before you install more than 1 anti-virus progs, install a recovery or restore utility such as GoBack to revert your computer back to a time when your computer was working just in case your computer should crash. You can download a 30-daytrial of GoBack from
www.download.com,
www.tucows.com, etc. or from Goback own site at
www.symantec.com, not roxio.com, because roxio sold Goback to symantec. And GoBack is the best revert utility on the market. If you would like to use it, disable WINDOWS XP system restore function first if you have XP (XP system restore pales in comparison because XP copycat cannot restore a system if the boot sector is damaged.)
Now, you mention your system may be freezing a lot. This problem stems from a lot of reason. You'll have to be more specific as to what is happening when your system freezes. :D 8)