I have a 2 yr old Dell E6400 laptop-- upon booting, getting a message that says something like, "cannot identify AC adapter-- adjusting accordingly." When Windows comes up, getting a message that says, "system is not communicating with battery." I didnt notice the battery to be very deficient prior to these messages, and I did have some virus problems prior to first getting these messages. Could this be caused by another virus, or do I need a new adapter, battery or something else?

This does happen on some of the chargers. Most times if you unplug power can let the battery drain all the way and plug back in it will fix its self. If this does not work it can be motherboard issue or adapter. See if you can try another one and if not it would be motherboard. When you try another adapter make sure it is at least 90 W. Hope this helps

2000-0142 and 2000-0146 are hard drive SMART errors, indicating the HDD will fail.
To boot message about the AC adapter was a BIOS level message that normally appears when a Auto/air ac adapter or other 65 watt ac adapter is used to power the system.

It's hard to say for sure, but it sounds like you might have a multiple issues going on. Are you able to boot the laptop with only battery power (the AC adapter not plugged in)? If so, then the culprit would be the AC adapter which you could order from Dell (or other places). If it still has issues even on battery power then there is something wrong with your operating system (Windows). Good luck.

Certain models talk to a chip in the adapter, sounds like this one has gone bad. At this point you can live with, buy a new one or pick up a used. This is a fairly popular model and you should be able to locate one from a dead laptop.

Try this first:

Use up the battery until the the screen would black out. Afterwards, reboot the laptop ( without using the AC adaptor) and the system will warn you of low battery - to press F1. After pressing F1, plugged the AC adaptor back and try rebooting.

If the above does not work, there is a Bios update from Dell that suppose to fix this problem.

If you need further help, reach me via phone at https://www.6ya.com/expert/elixirjose_00375ea24bd8141b

When you received the system as out of the box and you get this message "The AC power adapter wattage and type cannot be determind" let the Dell tech service know that they sent you a wrong or incorrect type of AC Adapter.
If this message not was not appearing before it just happened recently try to download and install the latest BIOS version from the support site.
If all fails remove the batt and check if laptop can work with AC adapter alone, because issues like AC not determine may not charge the battery and laptop may not work with AC as well, maybe laptop is already running on battery power.

Dell portable systems use a sensor wire (Thin wire in center of power connector of AC adapter) to allow the system to identify the wattage of the adapter connected to the system. If the wire is bent or broken, then that message will appear and the battery charger is disabled.
If the issue still occurs with a replacement or known good Dell AC adapter, replace the motherboard.

-check the system battery information and charger information of the computer; restart the computer, while the computer is booting up, press F2 repeatedly; check "battery info"
-if it stated ac adapter = unknown device ; update the BIOS (-visit support.dell.com
NOTE:other computers may have different BIOS, if it is not the same you may have to consult manufacturer
-click on drivers and downloads
-click or choose by service tag
-download the BIOS and install it on the computer)
-check information after updating; if same message AC adapter = unknown device; call manufaturer for a replacement AC adapter, if status has changed it is a battery problem or motherboard, contact manufacturer to test battery

Update the bios. I did it and problem solved.