Microsoft has decided that .hlp files are not the best option for your help files.
And you will probably receive an error message like: http://support.microsoft.com/kb/917607
If you still want to run your .hlp help files you still can look for a WinHelp viewer for Vista from Microsoft.
But as someone that has been in the application migration/upgrade bussiness for several year I think automatic migration is a very good option.
For example take a look at the following links:
Good luck! And If you had any other suggestions just leave a comment.