my laptop lags and crashes?

My laptop lags and crashes and I cant find the cause.?

I am currently using an 'Acer aspire 5742G'.
Here is a list fo the specs/software I use of the top of my head:
- windows 7 64bit
- microsoft office plus 2010
- used to use google chrome but now firefox
- spyware doctor with antivirus
- bittorrent
- divx web player along with divx media player etc
- adobe including the flash player etc

I bought my laptop a month ago. Most of the time it runs fine. Then it starts to lag, the audio (if playing music/videos) will lag too along with everything else. It takes forever for anything to happen.

After closing up all main programs and opening up task manager when nothing is running the pc usage is still high and going crazy (no stablility). When this happen I try to restart/shut down, it takes me to the "shutting down" screen but never shuts down.

I have scanned my pc for any viruses/malware/tracking cookies etc. but nothing comes up, my pc is absolutely clean.

It could probably be that certain softwares or registry items arent compatible or clash with one another but I have noidea how to pinpoint the problem.

Please help

ps whenever I open microsoft word/powerpoint for too long it really really lags the pc and crashes it especially when trying to save my data.