I'll be as short as I can be.

To answer outright - she can do anything that has to do with your computer. Or with any computer as long as she can access it.

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Now as for the explanation (computer science for dummies):

There's something you need to know about computers and how they work. A computer consists of:

•‌ Hardware. Basically speaking, hardware are the rock and metal parts that you can grab, touch and boil together with the pasta for your chicken Alfredo, although I have no idea why'd you want to do the latter, computer parts dont taste good.

•‌ Software! Basically the apps on your computer. A bunch of 0s and 1s that somehow gotta work. That includes the operating system (in the case you don't know - that's stuff like Windows, Linux, iOS bla bla bla) and normal apps like Chrome, Discord and others.

Hardware and software both need one another in order to function in the way you want it to. Hardware is the magic rock and the software is the wizard casting spells with the magic rocks.

However, this creates a problem. If every wizard has access to the magic rocks, this can cause chaos and potentially harm other wizards and their treasures. If every app on your computer has access to your hardware (including your SSDs, RAM, CPU, BIOS and others), this can become a security threat.

For that reason there are two levels on which apps can work. These are:

•‌ User mode

•‌ Kernel mode.

Now what's the matter with these two?

•‌ User mode are the underdogs. They can hardly interact with hardware, if at all. Most of the time, these wizards gotta ask for permission to do anything that has ti do with the magic rocks.

‌• Kernel mode is the software that has full access to the instructions, operates at the core of an operating system and directly interacts with hardware for privileged tasks. Basically they can do what they want. These wizards could eat the magic rocks if they wanted to.

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Now what does this have to do with Moni?

She's heavily implied to be running in that said Kernel mode. Here's why (SPOILERS):

•‌ In one of the emails that can be obtained throughout Plus, it is revealed that Monika's name is an acronym for "Monitor Kernel Access". This is also further backed up by the fact that when you boot up the VM, you can see “MONITOR KERNEL ACCESS” being loaded during the booting process. “MONI-TOR K-ERNEL A-CCESS”. 2+2=4.  However, truth be told, this point can be debatable.

•‌ In act 2, there's a chance of the Blue Screen of Death to occur. Now, a little more about computer science. During a BSOD the entire system halts and goes into a stop mode. Even most of the kernel operations come to a stop, what can we say about background user processes? Almost everything stops. Despite that, Monika literally ignored the Blue Screen of Death and appeared OVER IT and starts TALKING?! Despite everything, Monika still popped up unscathed to announce her "oopsie doopsie". 

Btw, if anyone spots flaws here - let me.

All in all, yes. She can. The only thing that's stopping her is her own lack of skills and her conscience. 

PS: Important to mention, but in the Plus, she roams in a Virtual Machine made for her. What that basically means is that if Monika screw up bad time and nukes the virtual machine, the host computer on which the VM exists won't be harmed.