which os to install

Solution
Windows 8.1 works a bit faster than Windows 7... so, Windows 8.1 would do better unless what causes Windows 8 to slowdown is not the OS itself but some issue related to malware, fragmented disk, startup programs, to many temp files, etc. Try runing CCleaner, defragging the disk, run a diskcheck (with CHKDSK C: /R typed on a command prompt window oppened as Administrator), and if you have the Windows installation disk, run a SFC /SCANNOW with the disk inserted in the optical drive, and also run a startup repair booting on to the installation disk.

You can run a SFC /SCANNOW even without an installation disk and it will ask for it only if needed... By experience I know it can do minor but important corrections to system file settings...
Windows 8.1 works a bit faster than Windows 7... so, Windows 8.1 would do better unless what causes Windows 8 to slowdown is not the OS itself but some issue related to malware, fragmented disk, startup programs, to many temp files, etc. Try runing CCleaner, defragging the disk, run a diskcheck (with CHKDSK C: /R typed on a command prompt window oppened as Administrator), and if you have the Windows installation disk, run a SFC /SCANNOW with the disk inserted in the optical drive, and also run a startup repair booting on to the installation disk.

You can run a SFC /SCANNOW even without an installation disk and it will ask for it only if needed... By experience I know it can do minor but important corrections to system file settings. I was experiencing some lagging issues weeks ago on Windows 7 after I removed the "My StartPage" bug from my browser and running sfc without installation disk made it regain speed and run like newly installed.
 
Solution
If your laptop is running slower than it use to when it was new, then consider backing up important data and doing a clean install of whatever OS the laptop came with; I assume Windows XP.

The first thing you need to do before upgrading to Win 7 or 8 is check the manufacturer's website to see if there are Win 7 or 8 compatible drivers. If there are none, then you may into issues after installing Win 7 / Win 8 like the wireless modem not working. If the manufacturer does have drivers for Win 7 / Win 8, then download them first before install the new OS.
 
Also clean up what you have, eliminate unused programs, clean up START UP, run disk cleanup, run defrag, if less than 4GB of DRAM consider a DRAM upgrade...be nice to know the laptop , how much DRAM etc