Looking for an app for multiple Android tablets to log into the same PC and enter data over the Internet simultaneously.
It sounds like you're trying to create a multi-user, web-enabled app; have you considered something like Google Docs (Google Docs FusionTables is something like a database), a shared Google Docs Spreadsheet, or something similar.
If you need to create your own application, and are all on the same LAN (local), you could set up XAMPP/WAMPP (a web server) on your own machine and build or install whatever you needed - your own private Intranet. There are lots of packages, and XAMPP includes MySQL, PHP, Perl or Python, and the Apache web server.
Can you give us some more information on what you are trying to do? I don't quite follow what you are trying to do. I'd like to visualise the situation more clearly before suggesting any solutions.
There are a couple of ways to try to interpret your statement. My initial impression is that you want to have a stand-alone spreadsheet, database, or other application on the PC that you want the tablet users to be able to access and update information, implying remote control of the PC from the tablets. This is possible, but not simultaneously. Imagine those same users standing around the PC attempting to enter their data through the single keyboard and mouse. It just isn't going to work well.
The other way is to have a client-server application. The PC will be the server part (although it can be a "regular" PC and not an expensive server) that will host the data store and possibly the business rules and other parts of the program as well as making sure that what one user is doing doesn't interfere with other users. The tablets will be the clients running an app that provides the "front-end" user interface, most likely does at least some data validation, and communicates to the server. In this way, the entire system is designed for multiple users to be entering, calculating, and retrieving data simultaneously and is far less likely to suffer any kind of data corruption. The problem here is that the system must be designed in this manner from the outset. You can't just take any existing program and make it work this way.
If you want a multi-user system, there are other things to consider. First, you need to have a secure, encrypted channel between the tablets and server either automatically created by the application or via the tablets connecting to the server over a VPN before starting the app. Second, client-server systems are more expensive with regard to licensing. Some may require a user license for each user, some may require a license for each simultaneous user, and others may just require a license for the server (rarely, but when that is the case, the cost is frequently based on the number of processors or processor cores on the server). Third, you need to have a well-documented and TESTED backup and disaster recovery plan for this system, especially if it is business-critical. Last, not all programs have multi-user versions available. What you want may need to be a customized version of something that is already out there or you may need to have the entire thing done as a bespoke system.