Hi
  I need to hide/disable fields based on the user groups when creating or editing items in the list. To be more precise some fields in the custom list should be disabled/hidden for users who are not part of admin user groups. Is there a way to do this by modifying the existing New and Edit form of the list?

There is no direct way you can achieve this in SharePoint. You cannot customize to this level from OOB. So, you might design the forms with custom code or you have to go for infopath forms.

Below are the ideas and work arounds on the web.

http://social.technet.microsoft.com/Forums/en/sharepoint2010programming/thread/680057dd-5ed4-4b94-b32a-9583e5d51f27

http://stackoverflow.com/questions/4444802/how-to-hide-field-based-on-current-user-role-in-sharepoint

http://www.sharepoint911.com/blogs/laura/Lists/Posts/Post.aspx?List=676af157-7d96-4e15-a987-54b8a3e4d948&ID=3

The last link doesn't work anymore. But I think this is the replacement for it:

Displaying SharePoint Fields by Permission Level
http://www.wonderlaura.com/Lists/Posts/Post.aspx?ID