Damp at floor level under window

[AJG]

Hello, i could do with advice please. Quick profile ... 2 bed inner terrace built 1901, kitchen and bathroom extension put on sometime in the mid 60's, i bought the house 1990, 3 years ago i had new double glazed windows throughout and a new back door (this is in kitchen next to lounge window) i decided to take out old skirting boards under living room window and discovered that they were mouldy behind and the wall in the recess under the boards was wet, i scraped out and hoovered up all of the loose rubbish and i have so far sprayed the wall and floor with a mould and mildew spray.
So, is there anything I can do myself? do I scrape all the damp and lose concrete and put some sort of sealent in there? if that's the answer can someone tell me which sealent would do the job - I'd be grateful for any suggestions please
Thanks
Alison

[welsh brickie]

buy a buitumin paint (synthproof) or similar and paint the wall and the floor around the area, also paint the back of the skirting and let it dry before fitting.