Not granting me access?

I'm running my PC on Windows XP. I've had it about 3 years now and it's the first time its started to play up.

Sometimes I'll click on Morzilla, Outlook Express or Internet Explorer (Checked that one to see if was all internet related icons and so far, it seems to be) and it keeps say it can't open it, perhaps I don't have permission to access these files.

In a few minutes it will work.

I fear I may have a virus or something but scans for this and spyware say my PC is clean (Infact it's the first time I've ever had no spyware what so ever on my PC, I usually have atleast 1 advertising thing on).

Can anyone help?

 

It's not fully updated. I'm currently involved in a rather heated arguement with the store where I bought my PC from. My XP is listed as not an offical copy even though I purchased it it as such, and now as a struggling uni student I can't afford to go out and buy a brand new copy of XP.

Thanks for the help