Firstly, it would help if you could tell us the error code the BSoD gave (or a screenshot) and uploaded the dump file (I don't know much about them personally, but for those who do know that's what they require). The devil is in the details, so the more you tell us, the easier it is to exorcise it.
You could probably also help yourself by looking at the task manager (Ctrl + Shift + Esc) when it's slow to see what processes are taking up the most resources. For example, one of my old laptops got plagued by "helpsvc.exe" taking up 98% resources after 5 minutes of running... I just kill that process when I see it, and problem solved. If nothing look unusual, then it may be one of the services (or hidden system processes) that's making it go slow.
That aside, possible causes: CPU fault, RAM fault, HDD fault, driver fault, viruses/malware (probably causing a memory leak that slows down your comp). It's most likely going to be either a HDD read error or a software fault (drivers, viruses, etc).