Enter safe mode as described above (post 5). Then follow this thread, and if you need to download anything use the mac to burn or copy to a flash drive for transfer. You will need to post the hjt and avg-antispyware log files as attachments in the security section, with a brief description of the problem encountered.
https://www.techspot.com/community/...lware-removal-preliminary-instructions.58138/
If you can't get into safe mode, you can borrow a WinXP cd with the same version of XP you have (home/pro sp1/2) and boot into it and select Repair. See this for instructions.
https://www.techspot.com/vb/showthread.php?t=8356&highlight=windows+repair
Hopefully that will enable you to get back to windows in one of the modes (normal or safe) and then run the removal instructions as linked.
If you're fast, you can try and abort it from logging out of windows by quickly pressing
start + run (shortcut for the Run command box) when it gets to desktop then typing in
shutdown.exe -a and enter.
Failing that, or if you wish to give up and start fresh, turn off the PC and unplug the HDD. Put it in a HDD enclosure or a IDE>USB adapter and then hook it up to your Mac to save any important data from it (and use a virus scanner to check those files being transfered) or if you wish, try and run the malware removal instructions from it (although some programs won't work for a mac, except the online scanners maybe) then you can plug in the HDD back into the PC and run the Dell System recovery disk (or Recovery partition >refer to your manual) and let it wipe the thing clean.
Your problem is probably related to some malware corrupting the OS preventing you from booting up the PC fully. The games will have to wait after you sort this issue out..