Oh, I don't recommend doing a reinstall on the assumption it may make windows load faster, it's likely there is an underlying problem here and a reinstall could possibly make it worse.
Something has changed and an attempt should be made to track it down first.
Check Device Manager for warnings, check Event Viewer for errors and warnings. Then run chkdsk to check for drive errors.
Also try to recall what has changed recently, added/removed any software, service packs, updates?
Are you using anti-virus, spyware, firewall software, if so which?
More details about your system would help.
Mobo, Ram, PSU, drives, video card...
