It sounds like you were originally set up as a local administrator. Now that you’ve joined a domain, are you still logging on as the local administrator, or a user on the domain? If you are logging onto the domain there may be a group policy in place that is preventing you from adding/removing programs. Check with your domain administrator to see if there’s a group policy in place on your domain. It’s also possible that there is a local policy set on that computer so you may want to check those settings as well.
Just an addition to this answer
you can check which GPO’s are assigned to that user on that computer using the GPMC (Group Policy Management Console) if you ask your domain admin to run a Group Policy Wizzard agains the user in question on the computer in question, it will then show all user applied and denied GPO’s and all Computer applied and denied GPO’s
GPMC can be downlaoded from MS and would need to run on a PC with .net 2 and the Admin pack for windows installed. found it works best on XP or windows server 2003.
1. Right click on My Computer
2. Click on manage
3. Click on Local Users and Groups -> Groups
4. Double click on Administrators
5. make sure your account is in this group.