Why does my laptop keep running slow then all of a sudden work fine?

i have an advent 9515 laptop running on vista home premium 32bit and lately it keeps completely tripping out to the extent of anything I open taking 5-10 minutes to open. Every time I turn on my laptop it seems to be a different problem, for instance i turned it on yesterday and looked on task manager as it was running really slow, it showed a lot of disk space being used and when i looked into it there was svchost.exe (local service network restricted) and in the column "write (b/min) it was upwards of 500,000,0 and the cpu was excessively high aswell. I turned off my laptop as i couldnt do anything with everything taking 5-10 mins to open, i looked on the internet on my phone for a sollution, when i turned on my laptop it was running fine. The day befor that when i started up my laptop it just wouldn't do anything at all so i pressed Ctrl Alt + Delete to open taks manager, after about 5 mins the screen went black and an error message popped up saying "failed. security options". things like this keep happening, i have ran scans for virus and spyware but it has shown nothing and i have performed a registry cleanup but these things keep happening. Any ideas apart from "get a new laptop" of whatthe problem might be and how to fix it?