Here is the lineage for Cnut. I could find no connection to William, even through the female lines.

Canute (Cnut) Svendsson, King of Denmark, Norway and England B: Abt 1000 D: 1035
Svend I "Forked Beard", King of Denmark, Norway and England B: ABT 960 D: 2 FEB 1014
Harald "The Blue Tooth" Gormsson, King of Denmark B: ABT 910 D: 1 NOV 987
Gorm "The Old" (Geva) Knudsson, King of Denmark B: Abt 840 D: Bet 920 and 940
Knud (Harde-Knud) Sigurdsson, King of Denmark B: Abt 814 D: 884
Sigurd "Snake-Eye" Regnarsson, King of Denmark B: ABt 786 D: 830
Ragnar "Lodbrok" Sigurdsson, King of Denmark B: 760 D: 845
Sigurd "The Ring" Randversson, King Of Denmark and Sweden B: Abt 730 D: 812
Randver Radbartsson, King Of Denmark B: Abt 670 D: ???
Radbart, King of Gardgarige B: Abt 638 D: Bef 690

As for Estrid, being Cnut's half sister, she is the only link between William and Cnut. Since it is a relation by wedlock and not a blood line it is a very weak link (but a link non the less). If i remember right this is the link used by William to claim the throne of England.

I hope this infomation was helpful.

Regnar