How can i check what is using up my memory on my computer?

I know a bit about computers and how they work and have a 2.6 ghz pentium 4 with 1gb ram and 160gb hard drive so is quick even though the standard is duo core with 2gb ram now days lol, but one thing i cant work out is how to check what could be slowing my computer down with memory, when i go into the task manager and go into performance i just see big spikes happening, and my cpu usage jumping from 70% to 90%, if i restart the computer it calms down and runs around 2% but after a while starts to pick up again, done virus checks and spyware checks, nothing there and deleted any programs that i dont use alot to free up the memory but nothing there

How can i check what is using up my memory on my computer?antivirus

Windows XP runs some maintenance tasks in background, that you want it to, generally, to speed up processing later. For example, after each boot, it will analyze the boot result, and move programs that autostart to better disk locations for faster access, if it needs to do so.

If you have disk indexing services enabled, Windows will also be building and refreshing indexes of your disk contents when it doesn't see you using your system heavily. When it does this, you'll see disk activity and high CPU utilization, particularly if you've been killing this process repeatedly for a long time. And if you've delayed previously scheduled maintenance tasks like disk defragmentation, your system resources will be used when the next scheduled task instance starts.