After 2 years my Vista (32-bit) machine now boots quite slowly.
The event viewer tells me two programs comming up slow: explorer.exe and svchost.exe.
Fine. But what can i do that these programs comes up as quickly as before ?
While you could try to investigate individual programs, and while there are measures to 'invigorate' your PC that can help, periodically it can be worthwhile re-building your machine; that is, re-installing Windows and then restoring your personal data from backup.
I probably average a rebuild around once a year, but then again, I am a techie...