The available memory is not what you think it would be. It not unused it really a file cache of recently terminated processes or trimed processes that have been force to give up some memory to other processes. They could be called back to there original purpose.
see for more detail.
http://support.microsoft.com/kb/312628
As to not have a page file this is very bad. Windows degradeS poorly without one. Remember even executable files are used as swap files when there is no page file.
Even if the drive is slow it better to have a page file until you get up to 8 to 16 gigs of memory. Some people think Even windows 7 can run without one then.
I regularly give old machine a boost by doing a few things. Clean up the hard drive as much as possible. Copy anything you can temporarily remover from the drive onto a backup.
Remove applications you don't need. Remove apps can reinstall.
When all that is done defragment your hard disk. At that point recreate your page file. It will be the closest to the front of the drive as is possible. Create a fixed size about 1.5 times memory. Thats my rule, usually I have seen sizes between 1 and 3 time memory. This will give it a slight boost in speed over the usual places it would be placed.
I use the auslogic defrager it's free (ads for more tool though). There are other that do this too. Check out the defragers at portableapps.com. It optimizes the disk by placeing recently accessed files near the front of the drive for faster access. It shows where the page file is placed so you can see if you moved it to the top 25% of the drive.
After that reinstall apps and copy back your data.
I would say you get 10 or 20% boost. But the main value is a lot of the hesitation goes away for a smoother experience.
