I had a horrible cold recently, first one in a year and it took about three weeks to clear rather than my usual three days. Sometimes I find I can shake off a cold completely if I hit it at the start with 500mg vitamin C every hour, liquid vitamin D and a good gargle with brandy, but sometimes you're just in for it, I'm afraid. My top tip is not going outside after dark. I've noticed that every time you do that with a cold it puts your recovery back by another day or so. No idea why - ozone maybe. And drink lots of lemon, honey and ginger. Ginger's magic.