I'm a little surprised that nobody's explained all the relevant points in one post.
1) If it mists up, it's no longer sealed. Therefore any demisting will only be temporary.
2) The cheapest 'solution' is to introduce breather holes, which will clear the misting, but reduce the thermal insulation significantly.
3) If the leaks can be identified and mostly sealed, desiccant like silica gel can be introduced (to buy extra time) and its access hole blocked.
4) By far the best solution is to have the window rebuilt with the original grass, but new flashing.
This isn't recommended by installers, as the UPVC required to surround the glass costs well under 2 pounds (US$3) a metre and a professional should be able to rebuild the window in minutes, leaving the customer to wonder why they're being charged a small fortune.
Even if they fill the cavity with some kind of dry gas from a cylinder, the additional expense would be minimal.
Many installers will be 'kind enough' to dispose of the old window for you, recycling the glass on their next astronomically expensive build. If the glass is recycled, there may be a faint line close to and parallel with one of the seals, indicating where the previous seal touched the glass.
One instance in which a new window might be justified is if you want to upgrade the glass. This could be for improved UV protection, higher reflectivity, solar power collection, increased strength or even bullet-proofing.
Anyone adept at DIY and with access to Google and YouTube should have no trouble doing the whole job in a morning. A friend did one of his years before the web and to the best of my knowledge it's been fine ever since.[/i]