What is the difference between currency manipulation, and a fixed exchange rate regime? China is labelled by some to be a 'currency manipulator', that is supposedly pegged to the US Dollar. What is the difference between currency manipulation and a fixed regime?

Different motives?
I figure that it may be that a textbook fixed regime is undertaken to prevent instability, and is minor (just to correct volatility), whereas manipulation is done with the aim of gaining an unfair advantage.

Different means?
I have also read that it could be due to the fact that a manipulator creates domestic base money with which it can buy currency, without sterilisation.

Legality? BVJ below makes an interesting comment about the legality of the two... is there a legal framework in which some forms of fixed currency maintenance are allowed and some not?

Could anyone please clarify? Thanks.
Correct, both a fixed exchange rate and a manipulated exchange rate involve the government setting this price. In BOTH cases it is usually implemented by promising to standing ready to buy or sell any amount at the fixed price. Of course, countries don't always have enough foreign cash to sell it at the promised price, nor do they always want to buy as much as the markets sell to them

The difference is, as you suggest, whether the trading partners (the US is usually the one that labels others manipulators) conclude that the exchange rate is fixed at a too depreciated level. This level keeps foreign goods cheap in the US and US goods expensive in the foreign country. 

Also, a fixed exchange rate is often set at a lvel that seems "too high", the only question being whether the government will manage to keep it so high, since it creates an incentive for its citizens to buy foreign goods. Instead a manipulated currency is deemed to be "too low".