Looking at the specs on the Acer site, that model has slots for 2 x SODIMMS, max 4GB - i.e. 2 x 2GB DDR3 modules.
Acer Support: Acer Aspire 5336 Notebook Series Specifications
Obviously, you've already got 2GB installed, so you just want to get 1 x 2GB
module of the correct spec.
I've got 4 x desktops and 1 x laptop at home, all running Win 7 Pro x64 happily in 4GB RAM. OK, none are used for games, but no-one in the family is complaining about poor performance!
Agree with what Milo says about disabling junk. You don't specify which model you have, single-core or dual-core Celeron. If it's a single core Celeron... Well, there's your problem, as they say.
SD