my laptop slow speed problem

Hi Friends,

I have

Dell inspiron 6400 model
Core 2 Duo 1.55 Ghz processor
2gb ram [dell's default hynix 1gb ram plus - 1gb other new ram]
Winxp [legal]

I dont have much startup items.. but after I start my laptop after some times it becomes slow..very slow.. sometimes it takes so much time that.. i have to wait for 2, 3 5 mins to open a new tab or window in browsers and start menu items

I tried increasing pageing file size but it didn't help!

Any idea what causing this problem?

Regards