You can upgrade the CPU, RAM, the WiFi card, the HDD/SSD, and the optical drive. The north bridge, front side bus, and socket limit your options for a CPU upgrade to the point where it's impractical, so throw that possibility out. The RAM is limited to 4GB by the CPU and bit size of the operating system if it is 32 bit. (right click computer, go to properties and look under system type) However if it is 64 bit you can use up to 16gb depending on your motherboard. (it is most likely 8gb max for you) This may help performance a little bit. The WiFi card only affects page loading speed, and is dependent on your router and internet service provider, this can speed it up a bit but it may be limited since Compaq is an HP company. (HP puts a lock on their systems) Putting in a bigger hard disk drive can speed up the system but I doubt it, putting in a solid state drive will always speed up any system but this can be upwards of $500 depending on the size. Upgrading the optical drive (dvd slot) will do nothing in terms of performance.
When it comes to laptops, you're better off upgrading, that's how they're designed. If you really wanted you could upgrade anything on it, but you'd have to unsolder things from the motherboard and find pieces that may be difficult if not almost impossible to find.
If you are willing to just buy a new laptop then check out Lenovo, they're the #1 PC manufacturer. Although you should wait a while, they're doing a refresh of their products.