xp problem

I am trying to install xp a vista preloaded acer aspire 5610, no floppy. After all the shrinking and partitioning, the xp wont start in 16 bit mode so have to load from command prompt in vista mode. It starts to load then blue screen of doom appears with error pci.sys. I've now decided to clean vista completely but I cant even do that as despite all my fiddling with the bios, I cannot boot from either usb hdd or fdd nor the dvd drive. Looks like vista is stuck here for good. Any suggestions.