How big?
MS has its CRM product, its Terminal Server, SMS and MOM server solutions there are third-party solutions for straight CRM from IBM and HP, Sun, and others as well.
SAP is quite strong, as is Oracle in the CRM market space.
As forum member jfp23 noted, in small settings you could use Terminal Services, Remote Desktop, Desktop Assistance, or some flavour of VNC.
It really would help if you refined your question.
Best wishes.
Bill Castner