Are pre-built PC's more vulnerable to malware?

Various factors can make computers more vulnerable to malware attacks, including defects in the operating system (OS) design, all of the computers on a network running the same OS, giving users too many permissions, or just because a computer runs on a particular operating system, such as Windows, for example.

Building a PC is great and all but right now there's a shortage of Graphics card and there's a shortage of AMD's new Ryzen 5000 series processors. There's also a shortage of Power Supply Units but that shortage isn't so bad. AMD is also still seeing a shortage of their 3000 series Ryzen Processors but it's to a lesser extent.

So if you want to build an Intel system with either Integrated Graphics or a really low end GPU like the GT 710 then go ahead. However, even Intel's weak integrated graphics are still better than a low end video card. Intel still makes decent processors.

Prebuilt PC's aren't any more or any less prone to malware, but some of the add-on bloatware programs they use are either borderline malware, or the programs are tracking you. But then again, facebook and google love to track people then sell the information at a profit. Also, you could just use the Windows 10 Media Creation tool and do a fresh install of the OS to the prebuilt PC. However, you will have to go to the manufacturer's website and download all the drivers for that PC. 

More vulnerable - No  (but keep in mind that no computer is invulnerable to malware if you don't use some common sense precautions and invite the stuff in)

Do they come preloaded with resource using OEM Proprietary Bloatware and Adware (sometimes thinly disguised as Trial Ware)? - Yes.

- and Companies like Google and Macro-Spy don't care if it is a pre-built or home built computer...they are equal opportunists when it comes to collecting and selling your data to advertisers if you let them. Google Chrome is a free browser because YOU and your info are the products being sold.

in theory, the major makers of pre-built machines do not do that.  They might, however, build in a way for the machine to be recovered from a catastrophe ... that's called a custom 'recovery' disk or program that can be run from USB or DVD.  [Windows 10 can make one for you as well, but most consumers don't bother making it until they need it -- after which it might not work well enough if the mobo drivers are what is corrupted].  In theory, the maker's software might be hacked before it gets to you -- and, they do test for this before releasing 'recovery' disks to owners.  NOTE:  a 'recovery' disk might not work if your machine is not registered with the maker -- this is a security feature designed to prevent thieves from accessing the machine after they steal it.  and, NO, these 'recovery' disks [both versions] will NOT grant access to the HDD is you have a password set.  If you set a password, there is little hope of ever recovering the contents if it is lost [or the machine stolen] .... the methods required take quite a long time to run [days at least] -- grampa

No. PC manufacturers don't intentionally install software that open back doors for malware. If ever, branded PCs have more reliable parts that are compatible with one another and software more secured since they are installed from trusted sources. Overall, it is the user who makes his PC vulnerable to malware.

Your friend is wrong. There is no evidence that prebuilt PCs have built in back doors making them vulnerable.

Additionally building your own means you have to deal with individual warranties on each item making any claim more complex.

As far as I know, Lenovo is the only company that got caught installing malware on new computers...and later admitted it, also.  

The real problem with prebuilt computer is that they are loaded with bloatware.  No matter who built the computer (you, or a factory) the very first thing you MUST do is to format the hard drive and install Windows.

That's the only way to safely get rid of the bloatware.

Many years ago, it was wise to build your own computer.  But now prices are outrageous for some components (power supplies and video cards in particular)  

So unless you have some strange hardware requirement that you can't order in a prebuilt...then yes, it probably would be best to order straight from Dell or HP.