Some users have had problems with Spybot installed on a machine running Avast.
I suggest you remove any S&D resident protection (Teatimer etc).
You may prefer to remove Spybot completely, and use the better performing MBAM instead. (If you do this, undo the immunity-if set- first. SpywareBlaster, by Javacool offers an excellent freeware alternative.)
Regarding updates, Avast typically updates the database once to three times daily, I can't speak for WD, and MS issue updates on the second Tuesday of every month. Occasionally (seems to be more frequently, recently) MS will issue an out-of-band update for a vulnerability, as happened last week.
It is a good idea to, as a minimum, have the security centre notify the user when updates are available. If you're a "hands-on" sort of user, you'll notice the yellow shield and manually download/install them when it suits.
If you don't want to be bothered with monitoring that level of detail (which is most users, I guess) set them to be automatically downloaded and installed.
Updates are considered important. They are usually patches for exploits/vulnerabilities. New vulnerabilities are constantly being found, if not patched, it increases the chance of malware installing. Every time MS software is installed, new updates for same are likely to be found, as most of the software is not downloaded/installed in an already-updated state.
There is not much point in using an AV unless the definitions are kept up to date. After a few days or weeks, it simply becomes the software equivalent of a paperweight. It will offer some protection, as Avast has fairly good heuristics, and new variants of old trojans might have some files detected and blocked, but the longer it's left without updates, the more holes it gets in it.