That's why the CPU is there - to do something - isn't it? Why do you want to have a CPU that doesn't do anything, just "sits there and waits" all the time?
Seriously, it depends on what you mean. In general, there is nothing wrong about using the CPU (for days, if you want). The only "problem" might be bad responsiveness of your computer when you are doing something else. For example, you are running an antivirus scan and writing some documents in Word simultaneously; if Word doesn't respond to your requests as fast as you need - and it does when the antivirus scan is not running - then you may say that it slows down your computer. (It doesn't actually have to be because of CPU usage, but e.g. because of disk access, but it doesn't really matter).
However, if you don't see any such slowdown (either because Word is responding fast enough, or simply because you don't do anything else when the antivirus scan is running), there is certainly no need to worry about CPU usage.
Also, the "responsiveness" of an application (e.g. Word) is probably more affected by the priority of the other process than by its CPU usage. You can lower the priority of an antivirus scan from avast! Simple User Interface by using the "Send to background" option. Processes with higher (normal) priority "have precedence" before the processes with lower priority, even it they (the ones with lower priority) use a lot of CPU.
So, what I'm trying to say is: don't worry about CPU usage, the only thing that really matters is your subjective feeling about the computer speed and responsiveness.
