Here is the torture: every night starting at about 7:30-8, it becomes slower and slower, stickier and sticker. I often have to turn it off just to get it to close everything down as it won't even do that. [...]
For now I just changed my time-date setting, just to say it's 3:42 am instead of pm.
Did that improve matters? If so, it could be you have some maintenance tasks scheduled for that time. You could look up your particular version of anti virus and Windows and see how to check for scheduled tasks. It could also be some malware using your computer, but since you've been running antivirus, let's hope not.
Do you know if you're running a firewall? Installing MS's free security products is a good start: http://www.microsoft.com/security/default.aspx
Do you run a lot of web applications? It's possible your residential network gets much busier during the evenings, but it doesn't sound like that's exactly what's happening.
You say you often have to turn it off; by that do you mean you cut the power to it before the operating system has shut down? That's bad to do to computers.
I think the first step is to make a complete backup. If you don't already have a way to do so, get a USB hard drive big enough to hold your entire system and some software such as Acronis True Image (which is what I use on my PC). Carefully follow the instructions.
The instructions for the following can be googled:
Once you have a backup, you can try installing all updates from Dell and Microsoft and your anti virus software maker (this all can take a while). Then disconnect the computer from the network and run a full virus scan. If that doesn't turn up anything, then run a full defragmentation of the hard drives. You can then connect to the Internet and try to update all your other software (browsers, etc.). If that doesn't fix anything, since you have a full backup, you could then try removing software which you installed but no longer use (be sure to properly reboot each time so each software package can remove all of its files).
If none of the above fixes the problem, it's probably time for something new. Macs are expensive to buy and own (ask me, I have two) and can become slower over time as well.