XPSP3 - It's Malware - I think?

Hi all, I need some help please..
(I do have access to W7, W10 and Linux (mint) on other machines, so no
"update your O/S advice" please!
I use Win XPpro SP3 (x32) for my daily driver and running all my now old
but very reliable peripherals, and up to now without any gotchas. 
I used to have NOD32 /ESET AV and did a scan with Malwarebytes
ocasionally but both now long discontinued for XP.
m/b GA-Z77-DS3H : processor iPentium lll Xeon :4GB ram. : 64GB SSD :1TB
HDD for data.
This system is shut down overnight and then rebooted every morning.
I can install from a backup but dont want to do that until I know what
has happened and cleaned up the system if possible. This is where I need
some help. 
Symptoms:
I suddenly noticed on morning boot a few days ago a couple of extra
screen flashes while XP was loading just after boot which made me
suspicious.
Websites I use all the time suddenly misbehaved - mainly loading
incorrectly or hanging. (I use myPal Browser which is the only updated
x32 browser I know of for XP nowadays).
However, if I use my VPN router which piggybacks on my main router then
all the webpages work perfectly if slightly slower than off the main
router, this would be an expected slowdown through the VPN connection. 
I have done a run on Hiram's 15.1 boot DVD with v.3 (I think)of mbam and
it found 4 PUM dropper instances on C drive (took hours for the 64 SSD
so not done on the 1TB HDD yet) but malwarebytes is very light on
information so I did not try cleaning those for the moment. Just 
continued using the system on the VPN for the present but for nothing
sensitive.
Does anyone have suggestions of bootable media that might be reliable
for checking these days to give me a cleanup pathway, I think I will
need to scan all the drives and anything USB I have plugged into this
machine recently. My guess is malware rather than virus but I am
uninformed and need help for the uninitiated! 
Thanks C+