Problem With Ispiron 6000 (Snail Slow)

Hello everybody. I'm new to the forum. I've always been a browser, and found alot of helpful information. Appreciate it much and in advance for this problem. Over the last couple of months, my Ispiron 6000 has been constantly slowing down. Now it is at about a near 10 minute (if not longer) boot and programs are taking at least 2 minutes to load up from the time I open them. That is on a good day. Everyonce in a while I get a speed spurt, but rarely. My battery has been giving me the critical signs of going out. I still have got to replace the battery, but I assume if running off of A/C power, it shouldn't affect the performance in this manner, right? I've been running McAfee and every trojan, etc. that has been located was removed. I've been reading up on examining my start up list and, well, I just don't know what to do. I've tried unchecking all of my start up items, leaving mcafee, to no avail. When I "apply" my changes to the start-up list, I always get some type of access denied error saying I need to switch to the admin account when I only have once (login) account on my computer. I need help and thank anyone who can assist me. The computer is even running slow in safe mode. ??? Jay