Is this something you've long been struggling with, that has been getting gradually worse over time? Or was your system relatively fast and just recently started lagging? If the latter, then you probably have a virus or spyware/adware program causing you trouble. If the former, then... well, there's a lot of things you'd need to do.
First and foremost, you'd want to consult the tutorial on
improving the performance of Vista. Note that there are some things you can do to narrow down the culprits, such as some services that may be an unwelcome load on your system. Also, you may be well overdue for a disk defragmentation. In any case, it's time to do some homework, or...
The simpler but more tedious approach is to back up all of your data and wipe your drive clean with a fresh install. Doing this clears out everything so you don't need to be concerned with remembering what you need or don't need. Then, with a clean operating system, you install only those programs you presently need/use. You'll very likely see a marked speed improvement.
Lastly, do you know how much RAM is installed and what the maximum amount is supported in your computer? For example, your PC can hold 4Gb of RAM and you've got just 1Gb installed. In this case, bumping up to even just 2Gb will deliver a marked improvement. Given how cheap memory is, I'd go with 4Gb.