Unable to install virtual center agent or hosts disconnected

Issue: While connecting 3.x hosts to the virtual center 2.5 you get an error saying unable to install virtual center server agent and/or incorrect username and password.
Set up: Virtual Center Server 2.5 Update x
Esx 3.x
When customer tries to add the existing esx 3.x host it fails with an error  “Unable to access file.
C:/program <file:///C:/program> files\VMware\Infrastructure\VirtualCenter Server\upgrade\vpx-upgrade-esx-7-linux-104215.sig.sig”

1. check whether you have the vpxa agent installed or not and if it is installed then what is the version of it.
2. rpm -qa grep vpxa
3. If the above command returns no output then skip to the step 6 and if it returns something like "VMware-vpxa-2.5.0-xxxxxx" then follow the next steps too.
4. service mgmt-vmware stop && service vmware-vpxa stop
5. now remove the existing vpxa agent (virtual center server agent) by the command                   rpm -e VMware-vpxa-2.5.0-xxxxxx
6.  In the vcenter server go to C:\Program Files\VMware\Infrastructure\VirtualCenter Server\upgrade and copy                                                                                                                       vpx-upgrade-esx-7-linux-xxxxxx and                                                                           vpx-upgrade-esx-7-linux-xxxxxx.sig                                                                                         to any directory of the troubled esx host and run the command                                                    sh vpx-upgrade-esx-7-linux-xxxxxx                                                                                          to install the vpxa agent (virtual center server agent).                                                                  Note:- xxxxxx should be the same as the build number of the vCenter server. If you do not find the above files then skip the next step.
7. If the above files are missing or still you are facing the same problem then download and install any Virtual Center 2.5 with a build number higher than the existing virtual center. example:- If your virtual center was 2.5 update 3 then you install virtual center 2.5 update 4 or above. Issue Resolved.
Thanks to :- http://vikashkumarroy.blogspot.com/2009/07/not-able-to-add-esx-host-into-virtual.html