Generally speaking you can use a PID to obtain a handle to a process with the OpenProcess functions. Using this handle you can call QueryFullProcessImageName to obtain the process name information. There are other ways to enumerate processes and match a pid to the enumeration.
In any event, there is no predictable or required correlation between an application name and an uninstallString in the registry. Don't forget that applications can be installed using different installers and they don't all use the same methods to install and uninstall applications.
Why do you want an Uninstallstring?