My guess is one of two things ....
lack of memory (but from what you say - it's been OK .... so maybe faulty memory?)
more likely .... your HDD is full
when windows starts up it creates a shed load of temporary files .... some for windows some are used as memory, some are used by the file system itself.
Without getting too techie
you need a minimum of 20-25% of the total size free disc space.
so a 100Gb hdd in effect (at best) has an effective capacity of 80Gb.
depending on how much memory you have fitted .... you might get away with exceeding that .... but best not to.