Well...cleaning the PSU with canned air...will probably be just as effective as paying someone $$ to do it for you.
If cleaning doesn't improve the situation, then you might consider just replacing the PSU. PSUs do fail and, next to hard drives, are probably the system component replaced most frequently (IMO).
<<I get a message: hardrive monitor detected an error (something like that)...>>
Please...understand...error messages are the best that the system can do to inform users/owners of known/perceived problems. The specifics of each error message...are important, if only because they help to eliminate wild guiessing on what the problem might be.
Others here can provide you with suggestions and guidance for testing your PSU...that don't cost anything other than a little of your time. I think that i would pay attention to those...before drifing into wishing and hoping that Best Buy can solve it.