My investments in my 401K have been in the most aggressive mutual funds that I have to select from. Yes, they are the most volatile when the market conditions are as they are now but they are the ones that can make you the most when the market is favorable. With the happenings going on now with the markets, yes, I am down to where I started the year in my 401K. Yes, it could likely go down more before it starts a rebound. However, my plans to use my 401K are years away and I'm not worried too much with this account for now.
I believe that you should put in as much as you can afford into you 401K as long as you have you home paid for and out of other long term debt. Also, never borrow from your 401K. I know of many friends that have become millionaires following this approach to their 401K's. Don't worry when the market falls. This is a buying opportunity. Review the options you have to invest in and look at the history of these funds. You'll find how many years they have been in the market with the number of years up and the number of years down. Don't fall for a money market account where you only earn 1 - 1.5% on you money.