Vostro 1500 extremely slow for certain tasks

I recently got a Vostro 1500 (2Ghz Intel Core 2 Duo, 3GB RAM, 120GB 7200RPM HDD), and all in all it's a great little laptop. But I've been having problems that are getting really annoying. Whenever I want to open a document in Word, Excel or whatever other office program, whatever window was open freezes for any time between 10 - 30 seconds, then the program will open, sometimes without the document I clicked on in the first place. If I go to open the program on its own, via the start menu or run this doesn't happen, it just runs instantly. And opening documents from within the program isn't slow either. It only occurs when I double click the document.
The only other places I've noticed this random freezing is if I click in an external link to open Internet Explorer (i.e. a link in MSN messenger, or the last.fm app for those that use it!), I get the same 10 - 30 second freeze, then it opens as normal. And in AutoPlay, if I insert a flash drive and go to "Open the folder to view files" option, same thing. It doesn't actually freeze, it just takes ages to open, and there's no indication that it's in the process of doing so; egg timer, HDD light flashing etc.
When I got the laptop I got it with XP Pro rather than Vista, and I had to reinstall it because I wanted to partition the drive. Anyone else have/had this problem? Or ideas what could be wrong? Thanks!