Christoph's right that regular 9's will tend to be too lose when you tune down to C...Go up to 10's and it should be more than fine...Go to 11's or twelves and you'd get a similar effect as having say 12 - 13 in standard tuning...
If you're having difficulty tuning, some things to consider...does your guitar have a floating bridge? If so, this will cause some initial problems when tuning, as the bridge has to find a "sweet spot" to stay in tune...A way of speeding up this situation is to drop all your strings before you start tuning...
Anywho, hope some of that was helpful...you've rekindled an old flame so I'm off to detune and rock out for a few hours, :D Cya later people!
Don't worry too much about me, ignore me long enough and I'll go away.
