It should take about 20 minutes to install! You must have a hardware problem.
There are lots of things to try but let’s firstly check if a particular program is slowing the machine when you switch on, go to Control Panel > All Control Panel Items > Performance Information and Tools > Advanced Tools (in the left pane). On this screen the problem is sometimes shown. If not, click View Performance Details in Event Log (Event Viewer). Events in the 100 series are boot events. These can be followed up by double-clicking them, then clicking Event Log Online at the bottom.
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