If you're saying this is happening in Windows, after you just installed it, I would turn my attention to possible hardware problems or incompatibilities, if something changed in the hardware or if the system is new.
If a computer is running slow, it could be overheating, a faulty hard-drive, bad BIOS configuration (try a CMOS Reset). If you have more than 1 partition, also try to format everything before install and install with no network cable, to exclude the possibility of this being a virus of some kind.
I would also try to make sure the installation media (USB, or DVD for example) is absolutely free of errors. Perhaps try with another DVD writer, or another image ? Or another burning software... I use CD Burner XP on Windows XP, 'cause it's free.