As the two previous answers rightly say, the sealed unit is blown. The main two reasons for this are 1) degradation due to age & 2) Not being fitted correctly in the first place, i.e. glazed unit not stood on packers to prevent water ingress from free standing water inside frame (usually blocked drainage channels) Additionally, rare occurences of externally glazed windows where glazing clips (barbed "one-way" clips) that prevent unauthorised removal of the glass have rusted and broken the seal. Repalcement sealed units vary greatly from supplier to supplier, but a ballpark figure in my area is £100 per sq/m fitted (multiply w x h x 117.5 for final cost) There is usually a minimum charge of 0.4m/sq. Hope this helps.
Kev@kl.plastics
p.s. you dont have to change the whole window, only the glass
Kev, October 2006