NEW YORK (CNN/Money) - I want to put about $100 every two weeks into some sort of investment that would be fairly liquid -- something I could have access to within a couple of days to week. What kind of investment should I be considering?

—Joanne, Durham, North Carolina

If you're really concerned only with liquidity -- the ability to turn your investment into ready cash relatively quickly -- then you've pretty much got the entire gamut of investing options to choose from.

Certainly, bank savings accounts, CDs and money market accounts work, in that you can go to your bank and have cold hard cash in your hands virtually any business day (although if you cash in a CD before it matures, you'll likely pay a penalty).

Money market mutual funds are also highly liquid since you can always write a check against your money-market account.

But stocks, bonds and stock and bond mutual funds also fill the bill quite nicely if converting them to cash quickly is your goal. Each of these investments can be sold on any day that the financial markets are open for trading.

I wonder, though, if what you're really concerned about isn't liquidity, but time horizon.

Let me explain.

Suppose you're saving for the down payment for a house that you intend to buy two years from now. Do you think it would be a good idea to put your money into stocks or stock mutual funds?

On the surface, that might not seem like a bad idea.

After all, over the past 12 months, the broad stock market has gained roughly 10.5 percent, while yields on more conservative investments like money-market funds and bank CDs have recently been running less than 3 percent.

Stocks are clearly a better deal -- no brainer, right?

Ah, but what happens if you plunk your down payment dough into a stock fund and a few weeks before closing on your new home, the stock market hits a big downdraft and loses, say, 20 percent?

You might not have enough cash to close the deal, which means you might have to postpone the purchase. Who knows, you might even lose some of the deposit you put up to take the house to contract.

Which brings me to a cardinal rule about investing: the sooner you'll need the money, the less aggressively you should invest.

In the case of our down payment example, the fact that you'll have to tap into your stash within two years means that you should probably be hunkering down in investments whose value doesn't fluctuate very much over the short term.

Basically, that means money-market funds, short-term CDs and, possibly, very short-term bond funds (say, funds with average maturities of two years or less).

Granted, the yields on these types of investments are pretty mingy these days.

But when you need to be sure that your funds are going to be available in a relatively short-period of time, stability and security are more important than return. (That said, you still want to get the best yield you can.

But what if you've got a longer timer horizon? Perhaps you're investing money for the college education of your kids who are now just starting grammar school. Or maybe you're stashing money away for your retirement, which won't occur for another decade or longer.

In that case, the short-term ups and downs of the stock market aren't a big issue.

You've got plenty of time to ride out setbacks in the market. What you're really want in this case is a shot at high long-term returns.

And when it comes to churning out lofty returns over long periods of time, stocks and stock mutual funds have it all over money funds and bank CDs.

But even if you've got a very long investing time horizon, I wouldn't recommend sticking all your moulah in stocks. I'd throw some of that money into bonds or bond funds as well, just to temper the up-and-down swings in the value of your portfolio a bit and to hedge against the possibility that stocks' future isn't as glorious as their past.

So before you can begin choosing specific investments, you've first got to think a bit about what your goals are -- what are you going to do with this money you're investing -- and how soon you are going to start tapping into your stash? Once you have a handle on those two key issues, you can focus on putting together a group of investments that can get you a decent return with an acceptable level of risk. For some guidance on how to do that, see the Asset Allocator.

Whatever investments you decide on, however, I definitely recommend that you stick with your plan of socking away that $100 every two weeks (and consider increasing it if you can).

Because when you come right down to it, although earning a good return is certainly an important aspect of building wealth, saving money on a regular basis is even more crucial -- and something you have a lot more control over.

Walter Updegrave is a senior editor at MONEY Magazine and is the author of "We're Not in Kansas Anymore: Strategies for Retiring Rich in a Totally Changed World."