There is no problem with using the default location as indicated above, as all of the Altiris solutions are mostly self contained (they share some infrastructure but rarely step on each other toes ;).
However I would question whether it is wise to put the helpdesk and DS together in the same system, as the use of one solution could impact the other.
Helpdesk can be quite demanding on resources (SQL, IIS) and DS can also have its needs (Depending on the jobs you run, AClients connected, etc).
In all cases, it can be done!